ACKNOWLEDGEMENTS
I would like to thank Dr. Clint W. Boal for his support throughout this study, and
Dr. Loren M. Smith and Dr. Nancy E. McIntyre for their suggestions and comments that
greatly improved this thesis. Thanks to C. L. Coffman, NRCS Unit Office in Lubbock,
local NRCS county personnel, and land owners in their support and assistance during this
study. Thanks, also, to my field assistants Pamela Newton, Josh Deal, and Lawson
Dennis, as well as all the volunteers who helped collect the data for this study. Funding
and support for this project were provided by Texas Parks & Wildlife, the USGS Texas
Cooperative Fish & Wildlife Research Unit, and Texas Tech University.
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS..........................................................................................
ii
ABSTRACT..................................................................................................................
v
LIST OF TABLES........................................................................................................
vii
LIST OF FIGURES ......................................................................................................
xii
CHAPTER
I.
INTRODUCTION ................................................................................
1
II.
BREEDING AND WINTERING SEASON GRASSLAND
BIRD ABUNDANCES AND HABITAT ASSOCIATIONS
AMONG THE PRINCIPLE CONSERVATION RESERVE
PROGRAM COVER TYPES IN THE SOUTHERN HIGH
PLAINS OF TEXAS.............................................................................
5
Introduction....................................................................................
5
Methods .........................................................................................
9
Results............................................................................................
14
Discussion......................................................................................
30
Management Implications .............................................................
37
NESTING BIOLOGY AND NEST SITE CHARACTERISTICS
OF GROUND-NESTING BIRDS IN THE PRINCIPLE
GRASSLAND COVER TYPES IN THE SOUTHERN
HIGH PLAINS OF TEXAS..................................................................
72
Introduction....................................................................................
72
Methods .........................................................................................
74
Results............................................................................................
79
Discussion......................................................................................
85
Management Implications .............................................................
90
III.
iii
LITERATURE CITED ................................................................................................. 110
APPENDIX:
SCIENTIFIC NAMES OF AVIAN AND PLANT SPECIES
CITED............................................................................................................... 118
iv
ABSTRACT
To increase the value of Conservation Reserve Program (CRP) fields as wildlife
habitat, the Farm Bill of 1996 required participants with existing CRP contracts in Texas
to re-seed 51% of existing CRP fields with native grasses. Additionally, all new CRP
contracts require 90% of the land enrolled to be in native grass species mixes. However,
the direct value of native seeding for grassland birds has not been evaluated fully in the
Southern High Plains of Texas. We examined bird use and productivity during the
breeding season, and bird use during the winter among 4 principle CRP cover types: (1)
Old World bluestem (OWB) (Bothriochlora ischaemum), (2) weeping lovegrass (WLG)
(Eragrostis curvula), (3) native grass mix (NAB) including buffalo grass (Buchloë
dactyloides), and (4) native grass mix without buffalo grass (NAA). For 2001 and 2002
breeding season, 12 avian species were observed in the study fields. Species number,
richness, and total abundances of birds did not differ between cover types or between
years for overall avian abundances during the breeding season and May surveys.
Grasshopper Sparrow (Ammodramus savannarum), Cassin’s Sparrow (Aimophila
cassinii), Mourning Dove (Zenaida macroura), and Western Meadowlark (Strunella
neglecta) accounted for more than 95% of all observations for the two years.
Grasshopper Sparrows were the most abundant species in all cover types except
NAA, being in roughly equal proportion to Cassin’s Sparrows in these fields.
Grasshopper Sparrow abundances were higher in NAB and OWB fields during both the
overall breeding season and May survey periods. Grasshopper Sparrows were associated
with fields that had low percent bare ground, high percent grass cover, low to moderate
vertical densities, and low overall vegetation heights. Cassin’s Sparrow abundances were
higher in NAA fields compared to NAB and OWB fields for both the overall breeding
season and May surveys. Cassin’s Sparrows were associated with fields with moderate
shrub densities (> 2 per 50 m2) and/ or moderate to high percent bare ground (> 30%).
Western Meadowlarks occurred in low abundances across all CRP cover types and did
not show any consistent relationship based on vegetation characteristics within cover
v
types. Mourning Doves occurred in low abundances across all cover types, but were more
abundant in introduced grass fields compared to native grass mix fields.
Nests of 7 species were found in CRP fields. Grasshopper Sparrow (29.7%),
Cassin’s Sparrow (23.7%), and Mourning Dove (38.1%) accounted for 91.5% of all nests
located. Grasshopper Sparrow nest densities were higher in NAB fields compared to
NAA fields. Cassin’s Sparrows were not found to nest in NAB fields and were equally
abundant among the other cover types. Overall more Cassin’s Sparrow nests were found
in NAA and range fields compared to other cover types. Mourning Dove preferred
introduced grass fields compared to native grass mixes, being higher in WLG and OWB
fields. Nest success estimates for all species were similar among cover types and between
CP types for the years combined.
Winter avian abundance, species richness, and number of species differed among
CRP cover types. Native grass mix CRPs supported more avian species and higher
abundances of species during the winter than introduced grass CRPs. Western
Meadowlark (36.9%), Savannah Sparrow (24.7%), and Horned Lark (32.5%) were the
most common species observed for the two years, accounting for 94.1% of all species.
Western Meadowlarks were common across all cover types, while Savannah Sparrows
and Horned Larks were more abundant on the native grass mix CRPs.
The native seeding requirement of CRP has likely contributed to abundances of
avian species within the Southern High Plains of Texas during the breeding and wintering
season. The more diverse native grass mixes in term of both species composition and
structure influences the use of these cover types by several different species throughout
the year. Results of this study provide a clearer understanding of the applicability of CRP
plantings as a conservation tool for grassland passerines.
vi
LIST OF TABLES
2.1.
2.2.
2.3.
2.4.
2.5.
2.6
2.7.
2.8.
Avian percent compositions and total numbers observed in CRP
cover types for the breeding season (May–July) 2001 and 2002
in the Southern High Plains of Texas ...............................................................
40
Avian percent compositions and total numbers observed in CRP
cover types for May 2001 and 2002 in the Southern High Plains
of Texas.............................................................................................................
42
Total number and average number of avian species on CRP fields
based on cover type and CP (conservation practice) type during the
breeding season (May–July) and May 2001, 2002, and years
combined in the Southern High Plains of Texas...............................................
44
Breeding season species abundance, richness, evenness, and
dominance means (x̄) in CRP fields based on cover type and
CP (conservation practice) type for 2001, 2002, and years
combined in the Southern High Plains of Texas...............................................
45
May species abundance, richness, evenness, and dominance
means (x̄) in CRP fields based on cover type and CP
(conservation practice) type for 2001, 2002, and years combined
in the Southern High Plains of Texas ...............................................................
46
Average breeding season (May–July) relative abundances for
the dominate avian species in CRP fields based on cover type
and CP (conservation practice) type for 2001, 2002, and years
combined in the Southern High Plains of Texas...............................................
47
Average breeding season (May–July) singing male relative
abundances for the dominate avian species in CRP fields based
on cover type and CP (conservation practice) type for 2001, 2002,
and years combined in the Southern High Plains of Texas ..............................
48
Average May relative abundances for the dominate avian
species in CRP fields based on cover type and CP (conservation
practice) type for 2001, 2002, and years combined in the Southern
High Plains of Texas.........................................................................................
49
vii
2.9.
Average May singing male relative abundances for the dominate
avian species in CRP fields based on cover type and CP
(conservation practice) type for 2001, 2002, and years combined
in the Southern High Plains of Texas ...............................................................
50
Summer (May–July) vegetative composition of CRP fields
based on cover type and CP (conservation practice) type in the
Southern High Plains of Texas (2001–2002)....................................................
51
Dominant grass species compositions and proportions among
CRP cover types in the Southern High Plains of Texas (2001–2002)..............
52
Most common forb and shrub species compositions among CRP
cover types in the Southern High Plains of Texas (2001–2002) ......................
53
Summer (May–July) vegetation characteristics of CRP fields
based on cover type and CP (conservation practice) type in the
Southern High Plains of Texas (2001–2002)....................................................
54
Summer (May–July) vegetation characteristics of CRP fields
based on cover type and CP (conservation practice) type for 2001
in the Southern High Plains of Texas ...............................................................
55
Summer (May–July) vegetation characteristics of CRP fields
based on cover type and CP (conservation practice) type for 2002
in the Southern High Plains of Texas ...............................................................
56
Correlation coefficients between bird abundances and vegetation
variables in CRP fields during the 2001 breeding season in
the Southern High Plains of Texas....................................................................
57
Correlation coefficients between bird abundances and vegetation
variables in CRP fields during the 2002 breeding season in
the Southern High Plains of Texas....................................................................
58
2.18. Correlation coefficients between bird abundances and vegetation
variables in CRP fields during the breeding season (2001–2002)
in the Southern High Plains of Texas ...............................................................
60
2.19. Correlation coefficients between bird abundances and vegetation
variables in CRP fields during May 2001 in the Southern
High Plains of Texas.........................................................................................
61
2.10.
2.11.
2.12.
2.13.
2.14.
2.15.
2.16.
2.17.
viii
2.20. Correlation coefficients between bird abundances and vegetation
variables in CRP fields during May 2002 in the Southern
High Plains of Texas.........................................................................................
62
2.21. Correlation coefficients between bird abundances and vegetation
variables in CRP fields during May (2001–2002) in the Southern
High Plains of Texas.........................................................................................
64
2.22.
Avian percent compositions and total numbers observed in CRP
cover types for the wintering season (January–February) 2002
and 2003 in the Southern High Plains of Texas................................................
65
Wintering season species abundance, richness, evenness, and
dominance means (x̄) in CRP fields based on cover type and
CP (conservation practice) type for 2002, 2003, and years
combined in the Southern High Plains of Texas...............................................
67
Average wintering season (January–February) relative
abundances for the dominate avian species in CRP fields based
on cover type and CP (conservation practice) type for 2002, 2003,
and years combined in the Southern High Plains of Texas ..............................
68
Winter vegetation characteristics of CRP fields based on cover
type and CP (conservation practice) type in the Southern High
Plains of Texas for 2002 ...................................................................................
69
2.26. Correlation coefficients between 2002 winter bird abundances and
vegetation variables from Winter 2002 and Summer (July) 2001
in CRP fields in the Southern High Plains of Texas.........................................
70
2.27. Correlation coefficients between 2003 winter bird abundances and
vegetation characteristics from Summer (July) 2002 on CRP fields
in the Southern High Plains of Texas ...............................................................
71
2.23.
2.24.
2.25.
3.1.
3.2.
Nesting species composition and the number of avian nests located
in CRP and native shortgrass prairie cover types in the Southern
High Plains of Texas (2001–2002) ...................................................................
92
Nesting species composition and number of avian nests located
in CRP and native shortgrass prairie cover types in the Southern
High Plains of Texas for 2001 and 2002 ..........................................................
93
ix
3.3.
Nest densities (n/ha) for nesting species in CRP and native
shortgrass prairie cover types for 2001, 2002, and years combined
in the Southern High Plains of Texas ...............................................................
95
3.4.
General nesting biology (x̄ + SE) of Grasshopper Sparrow, Cassin’s
Sparrow, and Mourning Dove in CRP fields and native shortgrass
prairie in the Southern High Plains of Texas, 2001–2002 ................................ 96
3.5.
Mayfield nest success of the most common ground nesting birds in
CRP fields and native shortgrass prairie in the Southern High Plains
of Texas, 2001–2002......................................................................................... 97
3.6.
Overall nest success for the most common nesting species in 5
principle cover types and CRP CP (conservation practice) types
(2001–2002 combined) in the Southern High Plains of Texas ......................... 98
3.7.
Nest site characteristics among the most common ground nesting
bird species in CRP fields and native shortgrass prairie in the
Southern High Plains of Texas, 2001–2002 ...................................................... 99
3.8.
Grasshopper Sparrow nest site characteristics between CP1 and
CP2 fields in the Southern High Plains of Texas, 2001–2002.......................... 100
3.9.
Grasshopper Sparrow nest site characteristics between successful
and depredated nests in CRP fields in the Southern High Plains
of Texas, 2001–2002......................................................................................... 101
3.10.
Grasshopper Sparrow nest site characteristics between successful
and depredated nests in CP1 and CP2 fields in the Southern
High Plains of Texas, 2001–2002..................................................................... 102
3.11.
Cassin’s Sparrow nest site characteristics between CP1, CP2, and
native shortgrass prairie in the Southern High Plains of Texas,
2001–2002......................................................................................................... 103
3.12.
Cassin’s Sparrow nest site characteristics between successful and
depredated nests in CRP fields and native shortgrass prairie
in the Southern High Plains of Texas, 2001–2002 ........................................... 104
3.13.
Cassin’s Sparrow nest site characteristics between successful and
depredated nests in CP1 and CP2 fields and native shortgrass
prairie in the Southern High Plains of Texas, 2001–2002 ................................ 105
x
3.14. Mourning Dove nest site characteristics between CP1 and CP2
fields in the Southern High Plains of Texas, 2001–2002.................................. 107
3.15. Mourning Dove nest site characteristics between successful and
depredated nests in CRP fields in the Southern High Plains of
Texas, 2001–2002 ............................................................................................. 108
3.16. Mourning Dove nest site characteristics between successful and
depredated nests in CP1 and CP2 fields in the Southern High
Plains of Texas, 2001–2002.............................................................................. 109
A.1.
Avian species codes, common names, and scientific names ............................ 119
A.2.
Plant species common and scientific names ..................................................... 120
xi
LIST OF FIGURES
2.1.
3.1.
3.2.
Location of counties and study fields within the Texas Southern
High Plains used in study, 2001–2003..............................................................
39
Location of counties and study fields within the Texas Southern
High Plains used in study, 2001–2003..............................................................
91
Nest initiation dates for ground-nesting birds in the principle
grassland cover types in the Southern High Plains of Texas,
2001–2003.........................................................................................................
94
xii
CHAPTER I
INTRODUCTION
The Conservation Reserve Program (CRP), Title XII of the1985 Food Security
Act (1985 Farm Bill), was originally enacted with the dual purpose of conserving soil and
water resources and reducing agricultural surplus (Bartlett 1988). Amendments to this
original program (the Food, Agriculture, Conservation, and Trade Act of 1990, and the
Federal Agriculture Improvement and Reform Act of 1996), have also recognized the
value of these areas in providing habitat for wildlife (Wildlife Management Institute
2000). CRP enrollment currently stands at 13.5 million hectares with approximately 80%
planted in grass cover (USDA 2003). Texas leads all states with a little over 1.6 million
hectares or 11.8% of the U.S. total, most of it within the panhandle and Southern High
Plains (USDA 2003). While CRP often does not restore the native prairie that once
existed in these areas, it does provide areas of vegetation more characteristic to natural
conditions than cropland, as well as adding to the remaining available prairie remnants.
Of the historic 1.7 million hectares of shortgrass prairie that once existed on the
High Plains of Texas only 20% remains (Samson and Knopf 1994). These prairies that
were once dominated by buffalograss (Buchloë dactyloides) and blue grama (Bouteloua
gracilis) have largely been converted to cropland (e.g. cotton, wheat, sorghum or corn),
excessively overgrazed, or invaded by mesquite (Prosopis glandulosa) and other invasive
species to a point that they no longer function as short- and mixed-grass prairie
ecosystems (Bolen et al. 1989, Samson and Knopf 1996). While this is less than the
approximate loss of 99% of the eastern and Midwestern tallgrass prairies, the short- and
mixed-grass prairie habitats remain critically threatened (Samson and Knopf 1994, Noss
et al. 1995).
Loss of this habitat, coupled with the fragmentation and degradation of the few
remaining patches, has had a dramatic negative impact on obligate grassland birds
(Askins 1993, Knopf 1994, Peterjohn and Sauer 1999). During the past quarter century,
grassland birds have experienced “steeper, more consistent, and more geographically
1
widespread population declines than any other behavioral or ecological guild in North
America” (Knopf 1994: 251). With the intensification of farming practices in the late
1950s through the mid 1980s the situation for grassland nesting birds became
considerably worse (Best et al. 1997, Brady 2000). Eight of the 13 grassland species that
showed declining population trends from the Breeding Bird Survey (BBS) 1966-1996
(Peterjohn and Sauer 1999) occur on the Southern High Plains at some time during the
year.
Since its initiation, numerous studies have documented the positive effects of the
CRP as breeding habitat for grassland birds. Johnson and Schwartz (1993a and 1993b)
observed that several grassland bird species in the Northern Great Plain, such as the
Grasshopper Sparrow (Ammodramus savannarum), Baird’s Sparrow (Ammodramus
bairdii) and Lark Bunting (Calamospiza melanocorys), that had declined in abundance
from 1966-1990 were common in CRP fields. In the Midwest, Best et al. (1997) found
nests of 33 species in CRP fields compared with only 10 in row-crop fields, and the
number of nests found was 13.5 times greater in CRP fields. Patterson and Best (1996)
concluded that the CRP has likely contributed to an increase in the abundance of many
bird species in central Iowa, inasmuch as the row-crop habitat that it replaced has lower
bird abundance and supports fewer nesting species. In Illinois, Herkert (1998) noted that
the availability of grassland habitat in the form of CRP fields has positively influenced
the population trends of Grasshopper Sparrows within that state. In the Southern High
Plains, Berthelsen et al. (1989) concluded that Ring-necked Pheasant populations
benefited from CRP fields due to increases in nesting cover, brood cover, and winter
habitat. Within the same region Berthelsen and Smith (1995) reported that breeding nongame grassland bird populations also benefited from CRP by providing large areas of
nesting and brood-rearing habitat.
Fewer studies have been done on bird use of CRP fields during winter, although
studies have indicated that wintering grassland birds, including those with declining
population trends, readily use these areas. In the Midwest, Best et al. (1998) reported
Ring-necked Pheasant, Northern Bobwhite (Colinus virginianus), American Tree
2
Sparrow (Spizella arborea), and Dark-eyed Junco (Junco hyemalis) being more abundant
and widespread on CRP fields compared to cropland, and speculated that the availability
of the CRP during the winter was beneficial to the habitat requirements of these species.
King and Savidge (1995) reported higher densities of birds on CRP fields compared to
croplands in southeastern Nebraska during winter presumably as a result of CRP having
more vegetative cover. Berthelsen (1989) determined that CRP fields in the Southern
High Plains provided high quality habitat for Western Meadowlark, Savannah Sparrow,
and Ring-necked Pheasants, and that availability of CRP likely promoted dispersal and
survival over the winter months.
While these studies looked primarily at the differences in abundance of grassland
birds between CRP fields (CP1—perennial introduced grasses and legumes, and CP2—
perennial native grasses) and croplands, few have looked at how well the different
seeding mixture within the different CRP conservation types, especially the native seed
mixes, affect grassland bird composition and abundance. The primary reasons is that in
the beginning the majority of CRP contracts were for CP1s, and even when native grass
were planted they are still largely planted as monocultures (e.g., Missouri and Nebraska:
switchgrass (Panicum virgatum)) or comprised only 2 species (e.g., Texas: blue grama/
sideoats grama (Bouteloua curtipendula) mix). Berthelsen and Smith (1995) and
Berthelsen (1989) looked at nest densities and wintering abundances, respectively, among
three cover types, one native mix and two native/exotic mixes, within the Southern High
Plains of Texas. They found nest densities to be similar among the cover types for all
species, however during the winter the blue grama/Kleingrass (Panicum antidotale) (BG/
K) native/exotic mix was found to have the greatest avian winter densities and number of
birds observed, as well as provided the best winter cover for Ring-necked Pheasants. In
Nebraska, King and Savidge (1995) detected no relationship between bird numbers and
vegetation diversity, while Delisle and Savidge (1997) found that certain species showed
a preference for either the CP1 or CP2 planting types. Davis and Duncan’s (1999) study
in Saskatchewan supported these findings. They found that vegetative structure is more
important than plant-species composition in grassland bird habitat selection, as crested
3
wheatgrass (Agropyron cristatum) pastures were as attractive as native pastures to many
of the species studied.
In an attempt to improve grassland habitat for wildlife, the Farm Bill of 1996
required participants with existing CPR contracts in Texas to re-seed 51% of CRP land
with native grasses. Furthermore, all new CRP contracts require native species seeding of
at least 90% of the area. However, the value of native seeded CRP fields in providing
breeding and wintering habitat for obligate grassland birds has not been assessed fully on
the Southern High Plains. To better understand the contributions and values of the
different seeding mixtures to grassland bird conservation, I tested whether avian
abundances and compositions, as well as vegetation characteristics differed among the
principle CRP cover types found in the Southern High Plains of Texas during the
breeding and wintering season. Additionally, I investigated whether avian nest densities
and nest success differed among the principle CRP cover types and, where possible,
native rangeland.
4
CHAPTER II
BREEDING AND WINTERING SEASON GRASSLAND BIRD ABUNDANCES
AND HABITAT ASSOCIATIONS AMONG THE PRINCIPLE
CONSERVATION RESERVE PROGRAM COVER TYPES IN
THE SOUTHERN HIGH PLAINS OF TEXAS
Introduction
The expanse of short- and mixed-grass prairie that once existed from central
Alberta south to the Southern High Plains of Texas has undergone extensive reduction
since the arrival of Europeans over 500 years ago(Samson and Knopf 1994). In the past
100 years most of these areas have seen a loss of native vegetation exceeding 80 percent
(Samson and Knopf 1994). While this is less than the approximate loss of 99% of the
eastern and Midwestern tallgrass prairies, the short- and mixed-grass prairie habitats
remain critically threatened (Samson and Knopf 1994, Noss et al. 1995). Of the historic
1.7 million hectares of shortgrass prairie that once existed on the High Plains of Texas
only 20% remains (Samson and Knopf 1994). These prairies that were once dominated
by buffalograss (Buchloë dactyloides) and blue grama (Bouteloua gracilis) have largely
been converted to cropland (e.g. cotton, wheat, sorghum or corn), excessively
overgrazed, or invaded by mesquite (Prosopis glandulosa) and other invasive species to a
point that they no longer function as short- and mixed-grass prairie ecosystems (Bolen et
al. 1989, Samson and Knopf 1996).
Loss of this habitat, coupled with the fragmentation and degradation of the
remaining patches, has had a dramatic negative impact on obligate grassland birds
(Askins 1993, Knopf 1994, Peterjohn and Sauer 1999). During the past quarter century,
grassland birds have experienced “steeper, more consistent, and more geographically
widespread population declines than any other behavioral or ecological guild in North
America” (Knopf 1994: 251). Eight of the 13 grassland species that showed declining
population trends from the Breeding Bird Survey (BBS) 1966-1996 (Peterjohn and Sauer
1999) occur on the Southern High Plains at some time during the year. Breeding species
5
included Ring-necked Pheasant (Phasianus colchicus), Horned Lark (Eremophila
alpestris), Grasshopper Sparrow (Ammodramus savannarum), Eastern Meadowlark
(Sturnella magna), Western Meadowlark (S. neglecta), and Cassin’s Sparrow (Aimophila
cassinii) (Oberholser 1974, Peterjohn and Sauer 1999, Kosteche et al. 2001, Seyffert
2001). Wintering birds include Vesper Sparrow (Pooecetes gramineus) and Savannah
Sparrow (Passerculus sandwichensis), in addition to the Ring-necked Pheasant, Horned
Lark, and Western Meadowlark (Oberholser 1974, Kosteche et al. 2001, Seyffert 2001).
If degradation of the grasslands has caused the decline of obligate grassland birds,
then the rehabilitation and restoration of these areas may help reverse these declines
(Koford 1999). The Conservation Reserve Program (CRP), Title XII of the1985 Food
Security Act (1985 Farm Bill), as well as the amendments to it (the Food, Agriculture,
Conservation, and Trade Act of 1990, and the Federal Agriculture Improvement and
Reform Act of 1996) allowed establishment of seeded (exotic and native) grasslands for
original contract periods of ten years. Originally enacted to conserve soil and water
resources, as well as to reduce agricultural surplus, CRP has gained recognition as
holding values as wildlife habitat (Wildlife Management Institute 2000). CRP enrollment
currently stands at 13.5 million hectares with approximately 80% planted in grass cover
(USDA 2003). Texas leads all states with a little over 1.6 million hectares or 11.8% of
the U.S. total, most of it within the panhandle and Southern High Plains (USDA 2003).
While CRP often does not restore the native prairie that once existed in these areas, it
does provide large areas of vegetation more characteristic to natural conditions than
cropland, as well as adding to the remaining natural prairie remnants.
Since its initiation, numerous studies have documented the positive effects of the
CRP as breeding habitat for grassland birds. Johnson and Schwartz (1993a and 1993b)
observed that several grassland bird species in the Northern Great Plain, such as the
Grasshopper Sparrow, Baird’s Sparrow (Ammodramus bairdii) and Lark Bunting
(Calamospiza melanocorys), that had declined in abundance from 1966-1990 were
common in CRP fields. In the Midwest, Best et al. (1997) found nests of 33 species in
CRP fields compared with only 10 in row-crop fields, and the number of nests found was
6
13.5 times greater in CRP fields. Patterson and Best (1996) concluded that CRP has
likely contributed to an increase in the abundance of many bird species in central Iowa,
inasmuch as the row-crop habitat that it replaced has lower bird abundance and supports
fewer nesting species. In Illinois, Herkert (1998) noted that the availability of grassland
habitat in the form of CRP fields has positively influenced the population trends of
Grasshopper Sparrows within that state. In the Southern High Plains, Berthelsen et al.
(1989) concluded that Ring-necked Pheasant populations benefited from CRP fields due
to increases in nesting cover, brood cover, and winter habitat. Within the same region
Berthelsen and Smith (1995) reported that breeding nongame grassland bird populations
also benefited from CRP by providing large areas of nesting habitat. By looking at the
reproductive rate of grassland birds in Missouri, McCoy et al. (1999) was able to identify
CRPs as source habitat for several grassland species, however other species were not
shown to benefit from the availability of CRP.
Fewer studies have been done on bird use of CRP fields during winter, although
studies have indicated that wintering grassland birds, including those with declining
population trends, readily use these areas. In the Midwest, Best et al. (1998) reported
Ring-necked Pheasant, Northern Bobwhite (Colinus virginianus), American Tree
Sparrow (Spizella arborea), and Dark-eyed Junco (Junco hyemalis) being more abundant
and widespread on CRP fields compared to cropland, concluding that the availability of
the CRP was beneficial to the habitat requirements of these species during winter. King
and Savidge (1995) reported higher densities of birds on CRP fields compared to
croplands in southeastern Nebraska during winter possibly as a result of CRP having
more vegetative cover. Berthelsen (1989) determined that CRP fields in the Southern
High Plains provided high quality habitat for Western Meadowlark, Savannah Sparrow,
and Ring-necked Pheasants, and that the availability of CRP likely promoted dispersal
and survival of the pheasant over the winter months.
While these studies looked primarily at differences in abundance of grassland
birds between CRP fields (CP1—perennial introduced grasses and legumes, and CP2—
perennial native grasses) and croplands, few have looked at how well the different
7
seeding mixture within the different CRP conservation types, especially native seed
mixes affect grassland bird composition and abundance. Berthelsen and Smith (1995) and
Berthelsen (1989) looked at nest densities and wintering abundances, respectively, among
three cover types, one native mix and two native/exotic mixes within the panhandle of
Texas. In this study, nest densities were similar among the cover types for all species,
however during the winter the blue grama/ Kleingrass (Panicum coloratum) (BG/ K)
native/exotic mix was found to have the greatest avian winter densities and number of
birds observed, as well as provide the best winter cover for pheasants. In Nebraska, King
and Savidge (1995) detected no relationship between bird numbers and vegetation
diversity, while Delisle and Savidge (1997) found that certain species showed a
preference for either the CP1 or CP2 planting types. Davis and Duncan’s (1999) study in
Saskatchewan supported these findings. They found that vegetative structure is more
important than plant-species composition in grassland bird habitat selection, as crested
wheatgrass (Agropyron cristatum) pastures were as attractive as native pastures to many
of the species studied.
During the first 3 years of the CRP, about 1.2 million hectares were enrolled in
Texas, 54.3 % in native grasses and 40.2% as introduced grasses (Smith et al. 1996). In
an attempt to improve grassland habitat for wildlife, the Farm Bill of 1996 required
participants with existing CPR contracts in Texas to re-seed 51% of CRP land with native
grasses. Furthermore, all new CRP contracts require native species seeding of at least
90% of the area at an added expense to the landowner. However, the value of native
seeded CRP fields in providing breeding and wintering habitat for obligate grassland
birds has not been assessed fully in the Southern High Plains. To better understand the
contributions and values of the different seeding mixtures to grassland bird conservation,
I tested whether avian abundances and compositions, as well as vegetation characteristics
differed among the principle CRP cover types found in the Southern High Plains. In
addition I looked at what vegetative characteristics among the cover types were
associated with bird use.
8
Methods
Study Area
I studied avian abundance and use within 4 principle CRP cover types found in
the Southern High Plains (SHP) of Texas. This portion of the High Plains, also known as
the Llano Estacado, straddles the Texas-New Mexico border and is bounded by the
Canadian River to the North and the Caprock Escarpment to the South and East
(Blackstock 1979). In Texas, this region is characterized by nearly flat to gently
undulating featureless plains that include numerous natural depressions known as playa
lakes (Blakely and Koos 1974, Blackstock 1979). Study fields were selected from the
south-central and south-western sections of the SHP within Hale, Lamb, Lubbock, and
Terry counties (Figure 2.1). Elevation within these counties ranges from 880 meters in
the east to 1200 meters in the west.
The area has a dry continental-steppe climate with mild winters and most rainfall
events occurring from April to October (Blakely and Koos 1974, Blackstock 1979).
Normal average temperature during summer is 24.5 o C with the hottest temperatures
being recorded in July and August (26.6 o C and 25.6 o C, respectively) (National Oceanic
and Atmospheric Administration [NOAA] 2003). Maximum highs during this period
often exceed 31.5 o C (NOAA 2003). Winters are generally mild with normal daily
temperatures for the 2 coldest months (January and February) at approximately 4.8 o C
(NOAA 2003). Annual average precipitation and temperature fluctuate widely ranging
from 22.1 cm to 102.9 cm, and -4o to above 50o C, respectively (Blakely and Koos 1974,
Blackstock 1979, Haukos and Smith 1997). The annual variability in rainfall and
temperature makes drought a frequent occurrence (Holliday 1991). Historically this area
was dominated by short- to mixed-grass prairie with large portions covered in
buffalograss and blue grama. Today the landscape is dominated by agriculture with large
portions of the study area counties now being used for cotton, grain sorghum, wheat,
corn, and other grain crop production.
9
Field Selection and Study Plots
Fourteen CRP fields were selected for study based on the original seed mixture,
the degree of stand establishment, and input from local USDA Natural Resource
Conservation Service (NRCS) personnel. The grass mixes chosen were those most
represented in the south-central portion of the SHP. These included 2 types of introduced
grasses (CP1s) and 2 types of native grass mix (CP2s). The CP1s were monocultures of
either ‘Old World’ bluestem (OWB) (Bothriochlora ischaemum) or weeping lovegrass
(WLG) (Eragrostis curvula) cover types, while the CP2s were a mix of 3 to 5 native
grass species. The principle grasses include in native mixes were sideoats grama, blue
grama, green sprangletop (Leptochloa dubia), switchgrass, and buffalograss. Native
mixes were differentiated into two cover types based on whether buffalograss was
included (NAB) or absent (NAA) from the native mix. Buffalo grass presence or absence
in the native seed mix was selected as a treatment because of its historical importance and
abundance in native shortgrass prairies, and because it is not always selected in native
mixes due to its cost and difficulty in getting established. Thus, 14 CRP fields selected
for this study consisted of 4 cover types, which included 3 WLG, 3 OWB, 4 NAB, and 4
NAA. All study fields were approximately 65 ha in area and separated by more than 0.5
km. CP1 fields were on average 7.3 years old, while CP2 fields were 4.3 years old.
Avian Abundance and Composition
We surveyed bird populations with fixed-radius point counts 3 times during the
breeding season (May, June, and July) (Reynolds et al. 1980, Hutto et al. 1986, Ralph et
al. 1993). Surveys started on 12 May in 2001 and 10 May in 2002. Each survey point was
a 75 m radius circle. All point count centers were at least 200 m apart to minimize the
possibility of counting the same birds in adjacent circles (Prescott et al. 1993). Within
roughly the middle of each study field of 65 ha, 9 fixed-radius point counts were
randomly established in a 3 x 3 arrangement that covered 30.25 ha (550 m x 550 m). In 2
fields I was only able to establish 6 fixed-radius point counts in a 2 x 3 arrangement. All
survey points were greater than 125 m from any field edge and at least 75 m from any
10
adjacent habitat type (e.g., playa lake) within the selected field to reduce the possibility of
edge effects during the counts. All birds seen or heard within the 75 m radius during a 5
min interval were recorded as well as any behavior, including singing by males, territorial
displays, perching substrates, etc. Birds flying overhead but not landing in or foraging
within the 75 m radius were not recorded. In addition, observers recorded the distance
from the center of the survey point to each detection in 5 m increments using a
rangefinder. This data was to be used to determine if there were differences in
detectability between the cover types for individual bird species. Surveys began half hour
before sunrise and ended approximately 3 hours after sunrise (Ralph et al. 1993). Surveys
were not conducted if it was raining or winds were > 18 km/hr to reduce the potential
bias that weather conditions might have on the counts (Martin and Conway 1994). Avian
abundances were expressed as the number of individuals observed per ha (no./ha) and
reflecting relative abundances and not actual densities.
We surveyed winter bird populations using fixed-width transects spaced 50 m
apart, with birds recorded within 25 m of either side of the line being transversed
(Burnham et al. 1980, Delisle and Savidge 1997). To reduce the potential biases
associated with this method, such as not detecting all birds within the transect strip or
inaccurately recording distance from the observer to the bird (Buckland et al. 2001),
transect width was kept small (Mikol 1980). The survey area was a 400 m x 400 m block
located approximately in the middle of each study plot established for the breeding
season surveys. Each block contained 8 parallel, 400-m-long transects (3.2 km/ study
field) yielding an area of 16 ha. I surveyed fields twice between January and February
throughout the day at least an hour and a half after sunrise or before sunset. In 2002
surveys started on 7 January and ended on 26 February, and in 2003 surveys started on 3
January and ended on 4 March. Surveys were not conducted if there was any
precipitation or winds were > 18 km/hr to reduce the potential bias that weather
conditions might have on the counts (Martin and Conway 1994). Avian abundances were
expressed as the number of individuals observed per ha (no./ha) and reflecting relative
abundances and not actual densities.
11
Vegetation Sampling
The vegetation of each study field was characterized twice during the growing
season (May and July). Each study field contained 3 transects spaced equidistantly with
the distance between transects depending on the size and shape of the field. Each transect
contained 5 equally spaced sampling points along its length, for a total of 15 sampling
points per field. Transects were greater than 50 m from field edges or from other habitat
types (e.g., playa lakes). At each sampling point, the structural density of the vegetation
was measured with a Robel pole (Robel et al. 1970). Readings were taken of the lowest
obstructed point on the Robel pole visible at a distance of 4 m and at a height of 1 m
above the ground for each of the 4 cardinal directions (Robel et al. 1970). The highest
vegetation intersecting or within 2 cm of the Robel pole in each of the 4 cardinal
directions was also recorded as a measure of average vegetation height. Percent canopy
coverage was visually estimated 1 m in front of the Robel pole and along the direction of
the transect being transversed using a modified 0.5 m2 Daubenmire frame (Daubenmire
1959). Percent cover of grasses, forbs, bare ground, and litter, as well as percent cover of
individual grass and forb species, and living and dead vegetation were determined on a
non-overlapping basis (i.e., the sum at a sample point could not exceed 100%). Maximum
heights of living, dead, grass, and forb were recorded from the vegetation with their bases
inside the Daubenmire frame. Standing dead vegetation was defined as any dead plant
material above the soil surface and still rooted in the ground (Best et al. 1997). Litter
depth was measured at the middle of each side of the Daubenmire frame and averaged
together. Litter was defined as any non-attached plant material lying flat on the soil
surface (Best et al. 1997).
We collected winter vegetation measurements only in 2002. Each study field was
sampled once between January and February within the perimeters of the 400 m x 400 m
winter survey plot. Three 400-m-long transects set 100 m apart were equidistantly
established inside each survey plot. Each transect contained 5 sampling points starting 50
m from the edge of the sampling point. Transects were spaced at 100 m intervals giving
each plot 15 sampling points. Measurements taken were similar to those taken during the
12
summer vegetation surveys, except living and dead canopy coverage and maximum
height were not recorded.
Data Analysis
Normality of avian abundance and vegetation experimental errors within
treatments was tested with the Shapiro-Wilk Normality Test (Shapiro and Wilk 1965) and
equality of variances within treatments was tested with Levene’s test (1960). Data were
normalized with natural log transformation as needed (e.g., percent grass cover, percent
forb cover, singing male Grasshopper Sparrows). If data could not be normalized, nonparametric test were used (see below) (Zar 1999). Avian data were separated into average
breeding season abundances (May, June and July surveys) and the initial May survey
abundances, both expressed as the number of birds per ha (no./ha). Avian data were
further separated out based on behavioral cues, in this case abundances of singing (i.e.,
territorial) males during the breeding season and the initial May survey. Singing male
abundance was used in addition to total abundance to discern breeding potential, defined
as occupation of a territory by a male (Johnson 1980, Hutto et al. 1986, Vickery et al.
1992). Berthelsen and Smith (1995) reported that 50% of all songbird nests located
during their study were initiated before 5 June, and during this study 76% of all nests
located were initiated before 10 June. In the absence of nest productivity data singing
male abundances might provide information about the quality of the habitat not detected
by looking at overall densities (Van Horne 1983 and Vickery et al. 1992). However, both
Van Horne (1983) and Vickery et al. (1992) concluded that high territory densities alone
is a misleading measure of habitat quality. Shannon’s (H’), Simpson’s (D), and Pielou’s
(J’) Indexes were used to estimate species richness, dominance, and evenness,
respectively (Magurran 1988, Krebs 1998).
I compared relative avian abundance, avian diversity, and vegetation variables
(dependent variables) among the 4 cover types and between the pooled CP types using
repeated measures analysis of variance (ANOVA) (Zar 1999). The whole plot factor was
cover type or CP type (independent variables) and year was the repeated measure.
13
Tukey’s HSD Test was used to identify differences among means when they were
significant (P < 0.05). If vegetation and avian abundance data could not be normalized,
Kruskal-Wallis one-way analysis was used to detect differences among cover types and
the Mann-Whitney U-test was used to detect differences among CP types (Conover 1980)
for years combined. Paired t-tests were used to detect differences between years within
cover types, and between 2001 and 2002 abundances for non-normal variables.
Pearson product-moment correlation analysis was used to determine relationships
between bird abundances and measured CRP vegetation variables. For species with
abundances that were non-normal, a Spearman rank correlation analysis was used.
Breeding season abundances and breeding season singing male abundances were
examined with the average vegetation means from May and July, whereas May
abundances and May singing male abundances were examined with only the May
vegetation variables. The wintering season abundances in 2002 were tested with the
corresponding vegetation measurements for that period. Additionally, I examined
wintering avian abundances from 2002 and 2003 with the summer vegetation variables
from the July prior to each of the winter avian surveys as another means for predicting
wintering season abundances.
Results
Overall Avian Abundance and Composition – Breeding Season
Sixteen species were observed in study fields during the breeding season (May –
July), 12 in 2001 and 12 in 2002 (Tables 2.1, 2.2, and 2.3). Ten species were recorded in
WLG and OWB fields, 9 species in NAB fields, and 11 species in NAA fields for the 2
years combined (Table 2.3). During the breeding season all study fields were dominated
by 1 or 2 species making up approximately 80% of all individuals detected for the 2 years
(Table 2.1). Average number of species did not differ among cover types (F3, 10 = 2.58, P
= 0.1117) or between years (F3, 10 = 2.25, P = 0.164), and there was no cover type x year
interaction (F3, 10 = 1.11, P = 0.3909) (Table 2.3). When cover types were pooled into
introduced grasses (CP1s) and native grass mixes (CP2s) a difference was detected (F1, 12
14
= 7.90, P = 0.0158), but there was no year (F1, 12 = 2.20, P = 0.1636) or CP type x year
interaction (F1, 12 = 1.03, P = 0.33) (Table 2.3). Overall CP1 fields had higher average
species number than CP2 fields. Breeding season bird abundances did not differ among
cover types (F3, 10 = 0.80, P = 0.522), between CP types (F1, 12 = 2.25, P = 0.159), or
between years (F1, 10 = 3.06, P = 0.1109 and F1, 12 = 3.27, P = 0.0958), and there was no
cover type (F 3,10 = 0.96, P = 0.448) or CP type (F 1, 12 = 1.76, P = 0.2096) x year
interaction (Table 2.4).
Species richness was similar among cover types (F3, 10 = 1.27, P = 0.3374),
between CPs (F1, 12 = 3.38, P = 0.0909), and between years (F1, 10 = 0.34, P = 0.5701, and
F1, 12 = 0.37, P = 0.5537) and there was no cover type (F 3, 10 = 0.38, P = 0.7677) or CP
type (F1, 12 = 0.01, P = 0.9424) x year interaction (Table 2.4). Species dominance and
evenness indices for the breeding season were similar among cover types, between CP
types, and between years, and there were no interactions.
Overall Avian Abundance and Composition–May Surveys
For the 2 years of this study I recorded a total of 14 species during May, 10 in
2001 and 11 in 2002 (Tables 2.2 and 2.3). WLG fields had 9 species, OWB fields had 8
species, and NAB and NAA fields each had 7 species total (Table 2.3). Fields were
dominated by 1 or 2 species accounting for greater than 75% of all observed individuals
for May (Table 2.2). Average number of species were similar among cover types (F3,10 =
0.27, P = 0.849), between CP types (F1,12 =0.27, P = 0.505), and between years (F1,10 =
0.77, P = 0.4011 and F1,12 = 0.82, P = 0.3828), and there were no cover type or CP type x
year interactions (F3,10 = 0.45, P = 0.7216 and F3,10 = 0.12, P = 0.7335, respectively) for
May (Table 2.3).
May bird abundances did not differ among cover types (F3, 10 = 2.30, P = 0.1395),
between CP types (F1, 12 = 0.19, P = 0.6672), or between years (F1, 10 = 2.98, P = 0.1148
and F1, 12= 3.34, P = 0.0926), and there was no cover type (F3,10 = 1.03, P = 0.4212) or
CP type (F1, 12 = 2.64, P = 0.1301) x year interactions (Table 2.5). Species richness,
dominance, and evenness indices for May were similar among cover types, between CP
15
types, and between years, and there was no cover type or CP x year interactions for any
of the diversity indices (Table 2.5).
Species Abundances—Breeding Season
Breeding season analyses were only performed on ground-nesting species
accounting for > 2% of all detections for years combined (Table 2.1). Grasshopper
Sparrow (67.1%), Cassin’s Sparrow (17.3%), Western Meadowlark (7.0%), and
Mourning Dove (2.7%) accounted for 94.1% of all detections for the 2 years (Table 2.1).
Grasshopper Sparrow was the most abundant species for all cover types except NAA
(Table 2.1). Breeding season abundances for this species did not differ among cover
types (F3, 10 = 2.86, P = 0.0906), between CP types (F1, 12 = 2.40, P = 0.1475), or between
years (F1, 10 = 2.25, P = 0.164 and F1, 12 = 1.9, P =0.1890), and there was no cover type
(F3, 10 = 1.41, P = 0.2961) or CP type (F1, 12 = 0.25, P = 0.6240) x year interaction (Table
2.6).
Cassin’s Sparrow was the most abundant species in NAA, and the second most
abundant in WLG and OWB cover types during the breeding season, accounting for
46.9%, 14.0%, and 12.2% of all detections, respectively (Table 2.1). Abundances did not
differ between years (U = 84.5, P = 0.5351), between years within cover types (WLG: U
= 4.0, P = 0.8273; OWB: U = 2.0, P = 0.2752; NAB: U = 4.0, P = 0.2482; NAA: U = 7.0,
P = 0.7728), or between years within CP types (CP1: U = 15.5, P = 0.6889; CP2: U =
26.0, P = 0.5286) (Table 2.6). When yearly breeding season abundances were averaged
differences were detected among cover types (H = 13.93, P = 0.003), but not among CP
types (U = 95.5, P = 0.98) (Table 2.6). NAA fields had higher average breeding season
abundances of Cassin’s Sparrows than NAB (P = 0.0014) and OWB fields (P = 0.0492),
but did not differ from WLG fields (P = 0.0763).
Western Meadowlark accounted for only 7% of all species detected during the
breeding season (Table 2.1). It was the second most abundant species detected in NAB
fields (11.6%), the third most in WLG and NAA fields (5.1% and 6.6%, respectively),
and the fourth most in OWB fields (4.2%). Relative abundances did not differ between
16
years (U = 84.5, P = 0.5351), between years within OWB (U = 2.0, P =0.2752) or NAA
(U = 5.0, P = 0.3865), or between years within CP types (CP1: U = 12.5, P = 0.3785;
CP2: U = 16.0, P = 0.0929) (Table 2.6). WLG (U = 1.0, P = 0.0495) and NAB (U = 1.0,
0.0433) fields differed between years each with higher abundances in 2002 than in 2001.
When years were averaged together there was no difference in overall breeding season
abundances among cover types (H = 2.78, P = 0.4271) or CP types (U = 73.5, P =
0.2954) (Table 2.6).
Mourning Dove was uncommon in all study fields during the breeding season
(Tables 2.1 and 2.6). Abundances did not differ between years (U = 95.0, P = 0.8904),
between years within cover types (WLG: U = 1.5, P = 0.1904; OWB: U = 4.0, P =
0.8273; NAB: U = 6.0, P = 0.5637; NAA: U = 7.5, P = 0.8852), or between years within
CPs (CP1: U = 12.0, P = 0.3367; CP2: U = 29.0, P = 0.7527) (Table 2.6). When yearly
abundances were averaged differences were detected among cover types (H =12.71, P =
0.0053) and among CP types (U = 27.0, P = 0.0007) (Table 2.6). OWB fields had higher
breeding season abundances than NAB study fields (P = 0.005), and CP1 fields had
higher overall abundances than CP2 fields (P < 0.01).
Singing Male Abundances–Breeding Season
There were no differences detected for Grasshopper Sparrow singing male
breeding abundances among cover types (F3, 10 = 3.07, P = 0.0779), between CP types
(F1, 12 = 2.26, P = 0.1588), or between years (F1, 10 = 0.29, P = 0.599 and F1, 12 = 0.28, P =
0.6039) (Table 2.7). Among the cover types, the difference in abundances between OWB
and NAA was not statistically significant (P = 0.0789), however the difference might be
biologically meaningful. Additionally, no cover type (F3, 10 = 1.19, P = 0.3615) or CP
type x year (F1, 12 = 1.07, P = 0.3208) interactions were detected.
Abundances of singing male Cassin’s sparrows during the breeding season did not
differ between years (U = 81.0, P = 0.4347), between years within cover types (WLG: U
= 3.5, P = 0.6625; OWB: U = 2.0, P = 0.2752; NAB: U = 4.0, P = 0.2482; NAA: U = 7.0,
P = 0.7728), or between years within CP types (CP1: U = 15.5, P = 0.6889; CP2: U =
17
26.0, P = 0.5286) (Table 2.7). For years averaged together, NAA had higher relative
abundances than other cover types (H = 14.27, P = 0.0026) (Table 2.7). Between CP
types for years averaged together no differences in singing male abundance was detected
(U = 93.5, P = 0.9065).
Breeding season singing male abundances for Western Meadowlark were similar
between years (U = 85.0, P = 0.5503), between years within WLG (U = 4.0, P = 0.8273)
OWB (U = 4.5, P = 1.0) and NAB (U = 6.0, P = 0.5637), and between years within CP
types (CP1: U = 17.0, P = 0.8728; CP2: U = 23.5, P = 0.3720) (Table 2.7). A difference
was detected in NAA between years (U = 1.0, P = 0.0433) with fewer singing males in
2002 than 2001. No differences were detected when yearly cover types were averaged
together (H = 2.79, P = 0.4271). However, when yearly CP types were averaged together,
CP2s had greater overall breeding season abundances of singing males than CP1s (U =
55.0, P = 0.0429).
Species Abundances—May
Grasshopper Sparrow was the most abundant species observed on all cover types
except NAA comprising 66.4% of all detections. May relative abundances for this species
differed among cover types (F3, 10 = 4.30, P = 0.0342), but no differences were detected
between years (F1, 10 = 3.30, P = 0.0992) and there was no cover type x year interaction
(F3, 10 = 0.81, P = 0.5145) (Table 2.8). Overall Grasshopper Sparrow May abundances
were higher in NAB (P = 0.0488) and OWB (P = 0.0493) cover types compared to NAA.
When cover types were pooled no differences were detected in abundance between the
CP types (F1, 12 = 1.23, P = 0.289), between years (F1, 12 = 3.20, P = 0.0987), and there
was no interaction between year and CP type (F1, 12 = 0.07, P = 0.7902) (Table 2.8).
Cassin’s Sparrow was the second most abundant species overall, and accounted
for 15.9% of all detections (Table 2.2). Among cover types, Cassin’s Sparrow was most
common on NAA fields and least on NAB fields. Abundances did not differ between
years (U = 84.0, P = 0.5201), between years within cover types (WLG: U = 3.5, P =
0.6625; OWB: U = 4.5, P = 1.0; NAB: U = 6.0, P = 0.5637; NAA: U = 5.0, P = 0.7237),
18
or between years within CP types (CP1: U = 17.5, P = 0.9362; CP2: U = 25.5, P =
0.4948) (Table 2.8). When yearly May abundances were averaged together differences
were detected among cover types (H = 10.34, P = 0.0159), but not among CP types (U =
91.0, P = 0.8200) (Table 2.8). NAA study fields had higher overall abundances of
Cassin’s Sparrows in May than either NAB (P = 0.0165) or OWB fields (P = 0.0341), but
they did not differ from WLG fields (P = 0.260).
Western Meadowlark was the third most abundant species overall accounting for
8.1% of all detections for the month of May (Table 2.2). Abundances were similar
between years (U = 65.0, P = 0.1295), between years within WLG (U =1.0, P = 0.1266),
OWB (U = 2.0, P = 0.2752) and NAA (U = 4.0, P = 0.4795), and between years within
CP1s (U = 17.5, P = 0.93629) (Table 2.8). Between years abundances differed for both
NAB (U = 1.0, P = 0.0209) and CP2 (U = 13.0, P = 0.046) fields. In 2002 abundances
were higher in both NAB and CP2 fields compared to 2001. When years were averaged
together no differences were detected in May Western Meadowlark abundances among
cover types (H = 1.46, P = 0.6923) or between CP types (U = 75.5, P = 0.2945).
Mourning Doves accounted for 2.1 % of the species detected in May (Table 2.2).
Abundances in May did not differ between years (U = 89.0, P = 0.6792), between years
within cover types (WLG: U = 4.0, P = 0.8273; OWB: U = 2.0, P = 0.2752; NAB: U =
6.0, P = 0.5637; NAA: U = 4.0, P = 0.4795), or between years within CPs (CP1: U =
13.5, P = 0.4712; CP2: U = 24, P = 0.4008) (Table 2.8). Yearly May abundances
averaged together did not differ among cover types (H = 7.30, P = 0.0630), but did differ
when pooled into CP types (U = 7.0, P = 0.0282). Among cover types the difference
between NAB and OWB (P = 0.0630) was not significant but possibly biologically
meaningful. Between CP types, introduced grass fields (CP1s) had higher abundances
than native grass mix fields (CP2s) when years were averaged together.
Singing Male Abundances – May
Singing male relative abundances for Grasshopper Sparrow during May did not
differ by cover type (F3, 10 = 3.55, P = 0.05565), CP type (F1, 12 = 1.59, P =0.2313), or
19
year (F1, 10 = 1.23, P = 0.2934 and F1, 10 = 1.39, P = 0.2612) (Table 2.9). However, the
differences between OWB and NAB when compared to NAA fields (P = 0.0820 and P =
0.0766, respectively) may be biologically meaningful. Additionally, there was no cover
type (F3,
10
= 0.24, P = 0.8675) or CP type (F1, 12, = 0.12, P = 0.7373) x year interaction
(Table 2.9).
May Cassin’s Sparrow singing male abundances did not differ between years (U =
86.5, P = 0.5972), between years within cover types (WLG: U = 3.0, P = 0.5127; OWB:
U = 4.0, P = 0.8273; NAB: U = 6.0, P = 0.5637; NAA: U = 5.0, P = 0.7237), or between
years within CP types (CP1: U = 15.0, P = 0.6310; CP2: U = 25.5, P = 0.4948) (Table
2.9). When years were averaged together a difference was detected among cover types (H
= 11.18, P = 0.0108). During May, abundances of singing males were higher in NAA
than both NAB (P = 0.0016) and OWB (P = 0.0157), while WLG study fields did not
differ between any of the other cover types (P > 0.05) (Table 2.9). Between CP types,
abundances were similar for years combined (U = 87.0, P = 0.6630).
During May Western Meadowlark singing male abundances did not differ
between years (U = 94.5, P = 0.8722), between years within cover types (WLG: U = 3.0,
P = 0.5127; OWB: U = 3.0, P = 0.5127; NAB: U = 4.0, P = 0.2482; NAA: U = 1.5, P =
0.1116), or between years within CP types (CP1: U = 17.5, P = 0.9362; CP2: U = 29.0, P
= 0.7527) (Table 2.9). When years were combined there was still no differences among
cover types (H = 5.49, P = 0.1392) or between CP types (U = 82.5, P = 0.5308).
Vegetation Characteristics–Breeding Season
The number of plant species differed among cover types (F3, 10 = 4.82, P =
0.0251) and CP types (F1, 12 = 6.58, P = 0.0248), but did not differ among years (F1, 10 =
1.38, P = 0.267) (Table 2.10 and Table 2.11). NAA study fields had a greater number of
plant species than WLG fields (P = 0.0166), whereas all other fields were similar in
number (Table 2.10). When grouped by CP type, CP2 fields had higher plant species
numbers than CP1 fields. Grass species number also differed among cover types (F3, 10 =
34.83, P < 0.0001) and between CP types (F1, 12 = 20.42, P = 0.0007), but did not differ
20
between years (F1, 12 = 1.84, P = 0.2004) (Table 2.10). WLG fields had the lowest number
of grass species compared to other cover types (P < 0.001). OWB and NAB fields were
similar in grass species number (P > 0.05), but these differed from NAA fields (P =
0.0111), which had the highest grass species number among all cover types. When cover
types were compared by CP type, CP2s had more grass species than CP1s (F1, 12 = 9.57, P
= 0.0093). CP1 fields were primarily monocultures of 1 grass species (> 90% of
vegetation cover), while CP2 fields were a mixture of 3 to 5 dominant grass species
(Table 2.11). The number of forb species did not differ among cover types (F 3, 10 = 0.84,
P = 0.5031), or between CP types (F1, 12 = 0.15, P = 0.7068), but did differ between years
(F1, 12 = 22.80, P < 0.001) (Table 2.10). In 2001 more forb species were recorded in the
CRP fields than in 2002. The number of shrub species did not differ among cover types
(F 3, 10 = 1.24, P = 0.3468), between CP types (F1, 12 = 2.44, P = 0.1445), or between years
(F1, 12 = 1.76, P = 0.1.988) (Table 2.10). Forb and shrub species compositions were
similar among cover types, however NAA fields had more species of both (Table 2.10
and 2.11).
Percent cover variables differed among cover types except for % forb cover and
litter depth (Table 2.12 – 2.14). NAA fields had the lowest grass coverage (F3, 10 = 28.96,
P < 0.0001), and higher living grass cover and lower dead grass cover than WLG study
fields (P = 0.0218 and P = 0.0074, respectively). Except for NAA, all cover types were
similar in % living, % dead, and % total grass cover (P > 0.05). Percent living (F1, 12 =
2.11, P = 0.1720), dead (F1, 12 = 2.78, P = 0.1213), and total grass cover (F1, 12 = 2.07, P =
0.1762) did not differ among CP types. Although percent forb cover did not differ among
the cover types (F1, 12 = 3.64, P = 0.0525), when these were grouped based on CP type,
CP1s had lower percent coverage in forbs than CP2 cover types (F1, 12 = 5.50, P =
0.0371). Total vegetative cover was lowest in NAA fields (F3, 10 = 22.62, P < 0.0001), but
when cover types were pooled into their respective CP types no differences were detected
(F1, 12 = 1.29, P = 0.2777). All vegetation cover variables increased from 2001 to 2002
except for percent living grass cover (Tables 2.13 and 2.14). Significant cover type x year
interactions occurred in NAB and NAA for percent living and dead grass cover. From
21
2001 to 2002 living grass cover decreased (F3, 10 = 15.71, P = 0.0004) and dead grass
cover increased (F3, 10 = 4.86, P = 0.0252) in NAB fields. In contrast, living grass cover
increased (F3, 10 = 4.01, P = 0.0251) in NAA study fields.
Percent litter differed among cover types (F3, 10 = 6.95, P = 0.0083) and between
years (F1, 10 = 16.50, P = 0.0023), but not between CP types (F1, 12 = 0.13, P = 0.7248)
(Tables 2.12 – 2.14). Litter decreased from 2001 to 2002, but there was no cover type x
year interaction (Tables 2.13 and 2.14). NAA and WLG study fields had higher percent
litter coverage than NAB study fields (P = 0.0076 and P = 0.034, respectively), while
OWB did not differ from the other cover types (P > 0.05). Litter depth was only
measured in 2002 and did not differ among cover types or between CP types (t12= 1.94, P
= 0.0758).
Percent bare ground differed among cover types (F3, 10 = 5.35, P = 0.0186) and
between years (F1, 10 = 7.18, P = 0.01861), but not between CP types (F1, 12 = 2.12, P =
0.1709) (Tables 2.12 – 2.14). NAA study fields had higher percent bare ground than
WLG (P = 0.0219) and NAB (P = 0.0376) fields. For all cover types bare ground
increased from 2001 to 2002 (Tables 2.13 – 2.14).
Visual obstruction readings (VOR) for both vertical density (VOR-lowest
obstructed) and average height (VOR-highest touching) differed among cover types (F3, 10
= 9.31, P = 0.003 and F3, 10 = 13.47, P < 0.001, respectively) and between CP types (F1, 12
= 28.93, P < 0.001 and F1, 12 = 10.64, P = 0.0068, respectively), but not between years
(F1, 10 = 0.29, P = 0.6022 and F1, 10 = 0.84, P = 0.3807) (Tables 2.12 – 2.14). Vertical
density was lower in NAA fields compared to CP1 cover types (P < 0.05), higher in
WLG compared to CP2 cover types (P < 0.05), and higher in OWB than NAA (P =
0.020). VOR average height readings for cover types were higher in WLG study fields
compared to all other cover types (P < 0.05). When cover types were pooled into CP
types, CP1s had higher VOR average height and vertical density readings than CP2s.
Total shrub density differed among cover types (F3, 10 = 6.51, P = 0.0102) but not
between CP types (t12= 1.35, P = 0.144) for 2002 (Tables 2.12 and 2.14). WLG and NAA
fields had higher densities of shrubs than either OWB or NAB study fields (P < 0.05).
22
Maximum grass heights differed among cover types (F3, 10 = 51.37, P < 0.001),
between CP types (F1, 12 = 7.51, P =0.0179), and between years (F1, 10 = 15.34, P = 0.0039
and F1, 12 = 13.84, P = 0.0029) (Table 2.12). WLG had higher maximum grass heights
than the other cover types (P < 0.001) for the 2 years, while between CP types CP1 fields
had taller grasses than CP2s. From 2001 to 2002, maximum grass height increased
among all cover types. A cover type x year interaction was only detected among OWB
study fields (P = 0.0175) increasing linearly from 2001 to 2002 (Table 2.13 and 2.14).
Maximum forb height did not differ among cover types (F3, 10 = 1.74, P = 0.2233) or
between CP types (F1, 12= 1.35, P = 0.2571), but did differ between years (F1, 3 = 6.43, P =
0.0295) increasing from 2001 to 2002. Both living and dead vegetation heights differed
among cover types (F3, 10 = 51.98, P < 0.0001 and F3, 10= 8.20, P = 0.0047, respectively)
and between CP types (F1, 12 = 15.54, P = 0.0006 and F1, 12 = 9.87, P = 0.0044,
respectively), but only dead height differed between years (F1, 10 = 17.85, P = 0.0018)
increasing from 2001 to 2002 (Table 2.12). WLG fields had the highest living vegetation
(P = 0.0002) among cover types, as well as higher dead vegetation than NAB and NAA
fields. When grouped into CP types, CP1s had higher living (P = 0.0109) and dead (P =
0.0081) vegetation compared to CP2 fields.
Relationships Between Breeding Bird Abundances
and CRP Vegetation Structure
In 2001 no significant relationships were detected between total breeding season
avian abundance and any measured vegetation variable (Table 2.15). However, in 2002
total avian abundance was negatively correlated with percent vegetation cover, percent
dead grass cover, vertical density, average vegetation height, maximum grass height, and
maximum dead vegetation height. Additionally, abundance was positively correlated with
percent forb cover and the number of grass species (Table 2.16). For the years combined,
maximum forb height was the only variable correlated with total avian abundance (Table
2.17).
23
The only vegetation variable Grasshopper Sparrow abundance was correlated to
for both 2001 and 2002 breeding seasons was percent bare ground cover (Tables 2.15 and
2.16). In both years total and singing male abundances were negatively correlated with
bare ground. In 2001 total and singing male abundances were both positively correlated
with vertical density. In 2002 both abundances were positively correlated with percent
vegetation cover, percent dead grass cover, maximum forb height, and maximum dead
vegetation height, while negatively correlated with the number of forb species present.
Only total abundance was negatively correlated with percent forb cover in 2002. For the
years combined, percent grass cover, and maximum forb and dead heights were
positively associated with Grasshopper Sparrow overall and singing male abundances,
while percent bare ground was the only vegetation variable that was negatively correlated
(Table 2.17).
Cassin’s Sparrow total and singing male abundances were negatively correlated
with percent vegetation cover in 2001 and 2002 (Tables 2.15 and 2.16). Additionally,
2001 abundances were positively correlated with shrub density, but only singing male
abundance was positively correlated with percent litter. In 2002 abundances were
positively correlated with percent bare ground, but only singing male abundance was
negatively correlated with percent dead grass cover. For the years combined, overall and
singing male abundances were negatively associated with percent grass cover and
maximum forb height. In addition, abundances were positively associated with percent
bare ground and shrub density (Table 2.17).
Western Meadowlark total abundance was positively correlated with percent
living grass cover in 2001 (Table 2.15). Additionally, 2001 singing male abundance was
positively correlated with percent native grass cover and negatively correlated with
vertical density. However, in 2002 no relationships were detected in breeding season
abundance. For the years combined, overall and singing male abundances showed a
negative relationship with maximum living height, while only singing male abundance
showed a positive association with percent native vegetation and a negative association
with vertical density (Table 2.17).
24
Breeding season abundance for the Mourning Dove was negatively correlated
with percent grass cover in 2001, and positively correlated with vertical density in 2002
(Tables 2.15 and 2.16). No other relationships were detected in 2001 or 2002. For the
years combined, Mourning Dove abundance was negatively correlated with percent
native vegetation and positively associated with vertical density (Table 2.17).
Relationships Between May Bird Abundances and CRP
Vegetation Structure
In 2001, total avian abundance was not correlated with any measure of vegetation
structure in May (Table 2.18). In 2002, total avian abundance was negatively correlated
with percent dead grass cover, percent vegetation cover, vertical density, average
vegetation height, maximum grass height, and maximum dead height, and positively
correlated with total plant species number and grass species number (Table 2.19). For the
years combined, total avian abundance was positively correlated with percent grass
cover, percent native grass cover, and maximum forb height (Table 2.20).
In 2001 and 2002, Grasshopper Sparrow May total abundance and singing male
abundance were positively correlated with percent grass cover and negatively correlated
with percent bare ground (Tables 2.18 and 2.19). In addition in 2001, Grasshopper
Sparrow total abundance was positively correlated with vertical density. In 2002,
Grasshopper Sparrow total abundance was also positively correlated with maximum forb
height, while singing male abundance was positively correlated with dead height. Both
measures of abundance were negatively correlated with the number of forb species
present. For the years combined, Grasshopper Sparrow abundances were positively
associated with percent grass cover and average vegetation height, while they were
negatively correlated with percent bare ground (Table 2.20). Only total abundance
showed a positive association with maximum forb height for the years combined.
Cassin’s Sparrow total abundance and singing male abundance were negatively
correlated with percent vegetation cover in 2001 and 2002 (Tables 2.18 and 2.19). In
2001, abundances were also positively correlated with shrub density and maximum grass
25
height. In 2002, total and singing male abundances were negatively correlated with
percent dead grass cover, but only singing male abundance was positively correlated with
bare ground. For the years combined, abundances were negatively correlated with percent
grass cover, vertical density, and maximum forb height. Only singing male abundances
showed a negative association with average vegetation height for the years combined.
Positive associations with Cassin’s Sparrow abundances included percent litter cover,
percent bare ground cover, and shrub density (Table 2.20).
Relationships between Western Meadowlark abundances and vegetation structure
were inconsistent between years and only for singing male abundance (Tables 2.18 and
2.19). In 2001, singing male abundance was negatively correlated with percent vegetation
cover, but 2002 abundance was positively related to this measure. Additionally, 2001
abundance was positively associated with the number of grass species, while 2002
abundance was negatively correlated with percent litter cover and positively correlated
with maximum forb height. For the years combined, maximum forb height was the only
variable positively correlated Western Meadowlark abundance (Table 2.20)
Low abundances of Mourning Dove throughout all study fields for May most
likely made it difficult to discern associations of this species based on characteristics of
the vegetation. However, abundance in 2001 were negatively correlated with percent
native grass cover (Table 2.18). For the years combined there were no significant
associations between abundance and measured vegetation variables (Table 2.20).
Overall Avian Abundance and Composition–Winter
Twelve species were recorded on study fields during the winters of 2002 and
2003 (Table 2.21). Western Meadowlark (36.9%), Savannah Sparrow (24.7%), and
Horned Lark (32.5%) were the most common species accounting for 94.1% of all
detections. Average number of species counted did not differ among cover types (F3, 10 =
1.66, P = 0.2384), but did between CP types (F1, 12 = 5.50, P = 0.3703) (Table 2.22). CP2
fields had higher average number of species than CP1 fields. The most common species
on CP1s were Western Meadowlark and Savannah Sparrow, while CP2 had these species
26
plus Horned Lark. No differences were detected between years for cover types and CP
types (F1, 10 = 0.12, P = 0.7315 and F1, 12 = 0.08, P = 0.7856, respectively), and there was
no cover type x year (F3, 10 = 3.63, P = 0.0536) or CP type x year (F1, 12 = 1.0, P = 0.34)
interactions. Total avian relative abundance differed among cover types (F3, 10 = 3.98, P =
0.0418) and between CP types (F1, 12 = 14.31, P = 0.0026), but not between years (F1, 10 =
2.63, P = 0.1358 and F1, 12 = 2.59, P = 0.1336). Among cover types (P < 0.05) and
between CP types (P = 0.0026) native grass fields had higher abundances than CP1
fields. Additionally, there was no cover type x year (F3, 10 = 0.90, P = 0.4759) or CP type
x year (F1, 12 = 0.49, P = 0.4991) interactions (Table 2.23). Species richness (H’) differed
among cover types (F3,10 = 4.49, P = 0.305) and between CP types (F1, 12 = 11.74, P =
0.005) with native grass fields being more diverse (P < 0.05) than introduced grass stands
(Table 2.22). Species richness did not differ between years (F1, 10 = 0.1156, P = 0.7409
and F1, 12 = 0.13, P = 0.72), and there was no cover type x year (F3, 10 = 0.11, P = 0.9549)
or CP type x year (F1, 12 = 0.02, P = 0.8988) interactions. Species evenness and
dominance indices did not differ among cover types, between CP types, or between years
(P > 0.05).
Species-Specific Abundances–Winter
Only those species consisting of greater than 5% of the total detected individuals
were included in the analysis of specific species abundances (Table 2.21). Western
Meadowlark, Savannah Sparrow, and Horned Lark were the only species that comprised
greater than 5% of the total detected individuals.
Winter abundances for Western Meadowlark were similar among cover types (F3,
10
= 0.4575, P = 0.711) and between CP types (F1, 12 = 1.06, P = 0.282), but did differ
between years (F1, 10 = 7.16, P = 0.0232), increasing in 2003 (Table 2.23). No cover type
x year (F3, 10 = 0.23, P = 0.8724) or CP type x year (F1, 12 = 0.14, P = 0.7120) interactions
were detected. Savannah Sparrow abundances did not differ among cover types (F3, 10 =
3.31, P = 0.0654). However the higher abundances in NAA fields compared to OWB
fields might be biologically meaningful (P = 0.0659). When cover types were grouped by
27
CP type, abundances were higher in CP2 fields compared to CP1 fields (F1, 12 = 6.36, P =
0.0268) (Table 2.23). No differences were detected between years (F3, 10= 3.75, P =
0.0814 and F1, 12 = 2.97, P = 0.1103) and there was no cover type x year (F3, 10 = 1.73, P =
0.2229) or CP type x year (F1, 12 = 0.04, P = 0.8432) interactions.
Horned Larks were found in all cover types except WLG. Abundances did not
differ between years (U = 87.5, P = 0.6295), between years within cover types (OWB: U
= 2.0, P = 0.2752; NAB: U = 6.0, P = 0.6587; NAA: U = 6.5, P = 0.7321), or between
years within CPs (CP1: U = 19.5, P = 0.812; CP2: U = 21.0, P = 0.7412). When years
were averaged together, NAB fields had higher abundances than OWB fields (H = 10.69,
P = 0.0296), and CP2 fields had higher abundances than CP1 fields (U = 38.0, P =
0.0071) (Table 2.23).
Vegetation Characteristics–Winter
Winter vegetation variables were collected only for the 2002 field season (Table
2.24). Forb cover (U = 5.32, P = 0.1499), bare ground cover (F3, 10 = 2.20, P = 0.1511),
litter depth (F3, 10 = 0.96, P = 0.4588), and shrub density (F3, 10 = 3.47, P = 0.0586) did not
differ among cover types. All other vegetation variables differed among cover types.
WLG, OWB, and NAB fields all had higher percent grass cover than NAA (F3, 10 =
22.96, P < 0.01). Litter depth was higher in WLG and NAA fields compared to NAB
fields (F3, 10 = 0.94, P < 0.05), but OWB fields did not differ from other cover types.
Vertical density (VOR-lowest obstructed) was higher in WLG compared to native seed
mixes, NAB and NAA (F3, 10 = 18.42, P < 0.001), but did not differ from OWB fields.
Average grass height (VOR-highest touching) was higher in introduced grass cover types
compared to native grass mixes (F3, 10 = 8.38, P = 0.0044). Maximum grass and forb
heights were higher in WLG fields compared to NAA, but there was no difference
detected among other cover types. Vertical density and average grass height were the
only vegetation variables that differed between CP types for winter 2002, with CP1s
having a higher vertical density and average grass height than the pooled native grass
mixes.
28
Relationships between Wintering Bird Abundances
and CRP Vegetation Structure
Because winter vegetation data were measured only in 2002 I could only test for
relationships between winter avian abundances and winter vegetation within that year.
However, I also tested winter avian abundances with the July (summer) vegetation
measurements from the previous year to determine if summer vegetation characteristics
could be related to the following winter’s avian abundances.
Only 2 of the vegetation measurements taken during winter of 2002 (Table 2.24)
were correlated with winter avian abundances (Table 2.25). Total winter avian abundance
and Horned Lark abundance were negatively related to winter litter depth. Additionally,
Horned Lark abundance was negatively related to average winter vegetation height.
When compared to 2001 July vegetation measurements, 4 significant relationships were
detected with 2002 winter avian abundance (Table 2.25). Total avian winter abundance
was negatively correlated to summer percent litter cover, winter Western Meadowlark
abundance was positively correlated to the number of forb species recorded in July, and
winter abundance of Horned Lark was negatively correlated with July’s vertical density
and average vegetation height. No significant relationships were detected between 2002
winter Savannah Sparrow abundance and winter 2002 or July 2001 vegetation variables.
Total winter avian abundance for 2003 was not correlated within any vegetation
variables measured in July 2002 (Table 2.26). However, winter abundances for the
Western Meadowlark, Savannah Sparrow, and Horned Lark all showed at least one
significant correlation with winter vegetation measures from the previous summer (Table
2.26). Western Meadowlark abundance was positively correlated with percent forb cover
and maximum forb height. Savannah Sparrow abundance was positively correlated with
the number of forb species, percent forb cover, and percent bare ground. Horned Lark
abundance was positively correlated with the number of forb species, and negatively
correlated with vertical density and average vegetation height from July.
29
Discussion
In the Southern High Plains of Texas the value of CRP established with native
grass mix is equal to that of CRP established with introduced grasses and is, in some
cases, better in providing breeding and wintering habitat for grassland birds. Furthermore,
floristic and vegetative diversity and the role they play in shaping the vegetative structure
within these CRP cover types are important factors in determining avian species
compositions and abundances.
During the breeding season introduced and native grass cover types did not differ
in total avian abundance, average number of species, or in species richness and diversity
indices. These results were consistent in regard to both the overall breeding season
averages and May counts, and suggest that breeding species diversity and abundances are
relatively consistent over the summer months. Pooling cover types into broad categories
based on CP type (CP1 and CP2) also reviled no differences in overall avian abundance
and diversity indicies. Similarly, studies in Nebraska (King and Savidge 1995, Delisle
and Savidge 1997), and Missouri (McCoy et al. 2001) detected no differences in total
avian abundance or species richness during the breeding season relative to CP type. CP2
fields in Nebraska and Missouri were for the most part monocultures of switchgrass,
whereas CP2 fields in this study were a mixture of 3 to 5 grass species with greater
potential at providing variable vegetation structure for grassland bird use. However, the
average number of species throughout the breeding season was higher on CP1 fields
compared to CP2 fields. Even though avian abundances and richness measures were not
different, means for these measures were consistently higher on CP1 fields. The
difference in average number of species and in species compositions between CP1 and
CP2 fields can be related to the varying vegetation structure within cover types and
individual CRP fields.
Grasshopper Sparrow, Cassin’s Sparrow, and to lesser extents Western
Meadowlark and Mourning Dove, were the most common species detected in CRP fields.
Grasshopper Sparrow and Cassin’s Sparrow accounted for 83.1% of all detections. Both
species have shown declining trends within the Central Shortgrass Prairie and Staked
30
Plains regions of the North American Breeding Bird Survey (Breeding Bird Survey
2003), and have been included on the “Migratory Nongame Birds of Management
Concern in the United States: the 1995 List” issued by the U.S. Fish and Wildlife Service
(USFW 1995). Among cover types species showed preferences toward specific plantings,
however no CP type preferences were detected for either species. Differences in cover
types and overall trends of species abundances (total vs. singing male) depending on the
survey period (May vs. breeding season average) were difficult to assess. Small sample
sizes, structural variability within CRP fields, and yearly fluctuations of species and
vegetation characteristics within cover types may have made differences in abundances
due to survey period or adult behavior difficult to discern within individual species
(Delisle and Savidge 1997).
Grasshopper Sparrow abundances were higher in NAB, OWB, and WLG fields,
but this species occurred only in half the NAA fields. A difference in abundance
depending on survey period or adult behavior was significant only among the total
abundance of this species during May. Grasshopper Sparrow abundances during May and
the breeding season were higher on NAB and OWB fields in both 2001 and 2002.
However, between CP types Grasshopper Sparrows did not show any preferences toward
CP1 or CP2 fields. Johnson and Schwartz (1993), Delisle and Savidge (1997) and McCoy
et al. (2001) also reported no differences in Grasshopper Sparrow abundances between
CP1 and CP2 fields. The only vegetation characteristics that were associated with
Grasshopper Sparrow abundances in both years were a negative relationship with percent
bare ground during the breeding season and May, as well as a positive relationship with
percent vegetation cover in May. The NAB, OWB, and WLG were all similar in percent
bare ground and grass cover and had relatively similar Grasshopper Sparrow abundances.
In North Dakota, Schneider (1998) detected higher abundances in areas of decreasing
bare ground, but also in areas of increasing litter. Litter cover was highest on NAA study
fields in this study, but this was also the study field with the greatest amount of bare
ground. Saab et al. (1995) recorded Grasshopper Sparrows in more lush vegetative areas
of the southwestern shortgrass prairie, and it has been speculated that they may be
31
heavily reliant on CRP fields in these areas for that reason (Vickery 1996). Patterson and
Best (1996) reported abundance was negatively correlated with vertical density of the
vegetation, with abundances higher in CRP fields with moderate grass height and vertical
density. In this study Grasshopper Sparrows were found in cover types with moderate
(NAB) to high (OWB) vertical densities and with low average vegetation height. The
differences between the 2 studies reflect the variations within CRP plantings between
geographic location (e.g., tall-grass prairie region of Iowa and shortgrass prairie region of
Texas). In Midwestern CRP, CP1 fields are normally the shorter, less dense stands, while
CP2 fields tend to be taller and more dense (Best et al. 1997). Such distinctions when
comparing CRP studies across geographic regions is critical to not misinterpret findings
concerning habitat preferences of species.
Cassin’s Sparrows were more abundant in NAA and WLG fields, and only
occurred in 1 OWB field and 2 NAB study fields. Like the Grasshopper Sparrow though,
these differences were dampened when cover types were pooled into CP type. The
Cassin’s Sparrow was the only species that showed consistent differences among cover
types for the different measures of abundance and survey period. Cassin’s sparrows were
more abundant on native mix without buffalograss (NAA) compared to OWB and NAB.
In 2001 and 2002, abundances were negatively associated with percent vegetation. In
addition, in 2001 abundances were positively associated with shrub density and grass
height, while in 2002 abundances were positively associated with bare ground and
negatively associated with percent dead grass cover. Maurer (1986) reported Cassin’s
Sparrows to be positively associated with percent bare ground cover in southeastern
Arizona. Ruth (2000) described Cassin’s Sparrow breeding habitat as a continuum of
shortgrass prairie interspersed to varying degrees with a shrub component and bare
ground. Territories of this species normally require a shrub component for song perches
and to initiate skylarking and flight songs from, in addition to enough grass cover to meet
nesting requirements (Maxwell 1979, Dunning 1999, Ruth 2000). These flight songs and
skylarking behavior have been hypothesized as serving the multiple functions of
territorial establishment, mate attraction, territory defense, and predator detection
32
(Anderson and Conway 2000). In areas that lack or have too few shrubs, as well as areas
with too high of densities and not enough grass cover, Cassin’s Sparrows will be absent
or in low densities, while habitats in between these 2 extremes are likely to meet the
species territorial requirements (Maxwell 1979, Dunning 1999, Ruth 2000). Within this
study, Cassin’s Sparrows were absent or in low abundances in NAB and OWB fields
were shrub densities were low and percent grass coverage was the highest, while NAA
and WLG fields met the habitat needs of this species to varying degrees and in different
ways. WLG fields had the highest shrub densities (x̄ = 12.24 + 5.99) among cover types,
while NAA fields had low to moderate shrub densities (x̄ = 2.44 + 0.87) but the most bare
ground of the cover types (x̄ = 42.71 + 10.66). In NAA fields where shrub densities were
< 2 per 50 m2, this species was observed to initiate their flight song from areas of bare
ground, and occasionally from the dead stems located at the tops of bunches of ‘Old
World’ bluestem and weeping love grass. This suggests that the Cassin’s Sparrow might
use areas with low shrub densities if they can substitute bare ground or other available
vegetation as song and flight song perches.
Western Meadowlark is another species detected during this study that also
showed a declining population trend on the Central Shortgrass and Staked Plains BBS
surveys (2002). This was the only species that occurred in all study fields in both 2001
and 2002. Abundances of Western Meadowlarks occurred in low but relatively equal
abundances among cover types and no differences were detected by survey period or
adult behavior during the breeding season. Between CP types, CP2 fields had higher
abundances of singing males during the breeding season than CP1s, but no other
differences were detected. Western Meadowlark abundances showed no consistent
relationships between vegetation characteristics within or between years. In Nebraska,
Western Meadowlarks were found to be equally abundant in native and introduced grass
CRPs (King and Savidge 1995, Delisle and Savidge 1997). Davis and Duncan (1999)
reported a similar pattern in Saskatchewn where Western Meadowlarks occurred in equal
frequencies in native pastures and introduced grass stands. Johnson and Schwartz (1993a)
found no significant relationships between abundances and vegetation variables within
33
the Northern Great Plains. In Iowa, Patterson and Best (1996) reported abundances to be
negatively correlated with vertical density and positively correlated to vertical patchiness,
while in Nebraska, Delisle and Savidge (1997) found abundances to be negatively
correlated with litter depth. These inconsistencies and discrepancies between vegetation
variables and abundances can partly be explained by the differences in location and
vegetation structure found in CRP within these different studies. But this, as well as the
low abundances reported in this study, can also be attributed to the generally large,
structurally diverse territories these species have (Weins 1969, Johnson and Schwartz
1993a, Patterson and Best 1996).
Mourning Dove abundances were low across all cover types and survey periods,
but CP1s had higher abundances than CP2s in both May and the breeding season. Within
cover types, NAB fields had lower abundances than the other cover types. Mourning
Doves did not show any strong associations for any of the vegetation characteristics
during this study. However, CP1 cover types had greater vertical densities, average
vegetation heights, and maximum grass heights than CP2s. George et al (1979) and
Houghes et al. (2002) noted that Mourning Doves nested in fields with tall live vegetation
characterized by tall clumps or bunches of grass in which they preferred to place their
nests under.
Winter avian abundance, species richness, and number of species differed among
cover types, as well as between CP types. Native grass CRPs supported more avian
species and abundances during winter than introduced grass CRPs. In Nebraska, Delisle
and Savidge (1997) reported bird abundances and species richness in CP2 fields as
almost twice that of CP1 fields. In contrast, McCoy et al. (2001) did not find any
differences in abundance and species richness between CPs. Total winter avian
abundance was found to be negatively associated with winter litter depth in 2002, and
negatively associated with the previous summers percent litter cover. Areas with less
litter cover and litter depth, more bare ground, and lower grass heights and vertical
densities might provide more favorable opportunities for foraging on the ground for seeds
(Best et al. 1998, Igl and Ballard 1999, Carrie et al. 2002). Additionally, grasses within
34
native seed mixes vary in seed size, production, timing of production, and energetic value
and may provide wintering granivorous grassland birds with more opportunities to forage
more efficiently. In addition, native grass mix CRPs within this study were still relatively
young (< 5 years) and contained considerable weedy species (i.e., forbs) that are
important food sources for wintering birds (Robel and Slade 1965). Klute et al. (1997)
determined that CRP fields in Kansas (which are mixes of native grasses) were better
seed producers than grazed native grass pastures and that these areas are important
sources of food for grassland birds during the winter. Local landscape features may also
play an important part in determining how wintering grassland birds utilize the CRP
cover types within the SHP. CRP that are positioned close to grain fields with residual
tillage, or those adjacent to playa lakes and the abundance of forb and weedy species that
these areas provide might be more utilized during the winter than CRPs that are not near
these features.
The Western Meadowlark, Savannah Sparrow, and Horned Lark were the most
abundant species recorded during the winter in this study. Western Meadowlark
abundances did not differ among cover types or between CP types. For 2002 no winter
vegetation characteristics were found to be correlated with meadowlark abundances,
although summer associations for both years were related to forb species and cover.
Savannah Sparrows were more abundant in CP2 fields, and somewhat in WLG fields.
Winter 2003 abundances were associated with summer 2002 forb species number,
percent forb cover, and percent bare ground. Horned Larks were more abundant on native
grass mixes and corresponds to this species preference for short sparse areas of bare
ground to forage in (Beason 1995). Abundances in 2003 were negatively associated with
winter litter depth and average vegetation height. Likewise summer vegetation
associations for both winter surveys included negative correlations with vertical density
and average vegetation height.
Besides vegetation compositions and the structural characteristics associated with
CRP cover types, other factors such field age and successional changes within fields
(Millenbah et al. 1996), availability and abundances of invertebrate prey or seed sources
35
(Hull et al. 1996, Klute et al. 1997, McIntyre and Thompson 2003), local and region
landscape features (Bakker et al. 2002, Fletcher and Koford 2002), and fragmentation of
the surrounding landscape compositions (Coppedge et al. 2001, Johnson 2001, Johnson
and Igl 2001) might also affect the distribution and abundances of grassland birds in the
SHP during the breeding and wintering season. Millenbah et al. (1996) reported that
younger fields (1-2 years since establishment) had higher avian diversities and
abundances than older CRPs (> 3 years since establishment) but higher productivity. CRP
fields within the current study varied in age among and within the cover types, and thus
between CP types. Average field age among cover types were: OWB and WLG field age
7.3 years, NAB 5.25, and NAA 3.25. Even with these differences cover types did not
differ in total avian abundance or diversity indices during the breeding season. However,
during winter the younger CP2 fields had higher abundances and diversity indices. These
early successional fields, besides containing a variety of grass seed sources, might also
provide additional sources of food from pioneering weedy plant species, as well as more
bare ground for foraging.
The availability and abundance of insectivorous prey during the breeding season
is critical in grassland nesting birds being able to establish a nest and to successfully
fledge young from it (Weins and Rottenberry 1979). In 2001 CRP fields used in this
study were sampled for arthropod richness and abundances to determine if there were
differences among cover types (McIntyre and Thompson 2003). Results from this study
did not detect any differences among cover types in relation to arthropod richness and
diversity. While further research is warranted, initial findings indicate that exotic grass
CRPs provide equivocal abundances and diversities of arthropod prey for grassland birds
during the summer (McIntyre and Thompson 2003). A similar study in Kansas (Hull et
al. 1996) found no relationship between bird abundances and invertebrate biomass within
native mix CRP that varied in forb abundances. These studies would suggest that
breeding grassland birds are not limited by prey abundances and diversities within CRP
fields regardless of grass mixture or forb abundance.
36
Numerous studies over the last couple of years have started to investigate how
local landscape and habitat features and composition interact to determine the distribution
and abundance of species (i.e., through fragmentation, disturbance, degradation)
(Coppedge et al. 2001, Johnson 2001, Johnson and Igl 2001, Bakker et al. 2002, Fletcher
and Koford 2002). These studies have demonstrated that local features do influence local
abundances and diversities of grassland birds species, and that it is important to recognize
how these feature within the landscape influence grassland bird use. Landscape features
that were associated with (< 400 m distance from) CRP fields within this study include
playa lakes (11 fields), other CRP (12 fields), woody edge (2 fields), cotton (7 fields)
other cultivated fields (4 fields), power lines (9 fields), and grazed native grassland (1
field). These features, depending on the species and time year could potentially affect the
way different CRP cover types are utilized by grassland birds.
Management Implications
Since its initiation in 1986 the Conservation Reserve Program has benefited
grassland birds in the SHP of Texas. CRP has provided large blocks of nesting and
brood-rearing habitat during the breeding season and alternative areas for cover and
forage in winter for non-game and upland game birds, as well as waterfowl, where
previously there was only cropland (Berthelsen 1989). But since 1997 with the start of
planting exclusively native seed mixes, CRPs have become of even greater value to these
avian groups that use grassland. During the breeding season, native grass mix CRPs had
equal to higher avian abundances and diversity indices compared to CRP planted with
exotic grasses. However, during the wintering season CP2s were superior to CP1 fields in
avian abundances and diversity indices. Additionally, CRPs planted with mixtures of
native grass and forb species will likely promote increased avian use and production
compared to monocultures of exotic grass stands (Delisle and Savidge 1997, Davis and
Duncan 1999, McCoy et al. 2001)
During this study pooling of cover types into the broad categories of CP1s (exotic
grasses) and CP2s (native grass mixes) often tended to dampen or occlude biologically
37
meaningful results (e.g., both the Grasshopper Sparrow and Cassin’s Sparrow). For this
reason the broad categories based solely on CP type alone should be avoided when
assessing the value of different CRP cover types. Vegetation structure and vegetative
composition in so far as helps to determine this structure likely influences how grassland
birds utilize CRP fields during the breeding and wintering season. Native grass mixes and
the individual grass species in these mixes, with their differences in germination rate,
growth form, production, successional development, and intra- and interspecific
interactions with other plant species, are more likely to provide and meet the requirement
of more grassland birds due to the variability created in vegetative composition and
structure than monocultures of exotic grasses.
Stands of native grass mixes, as well as the residual exotic grass CRP could be
maintained and enhanced through the use of management practices that simulate natural
historic disturbances and within grasslands (i.e., grazing, burning, mowing, discing).
These management practices should be further explored and investigated, especially in
light of planting native grass species that evolved with these disturbances (e.g., grazing of
buffalograss and blue grama). Likewise, control of weedy species, including certain
shrub species, by farmers should be reassessed in light of the potential use of these areas
by grassland species. Management of CRPs that support more heterogeneity within fields
(Fuhlendorf and Engle 2001) through variation in planting mixtures and use of limited
management practices to maintain and enhance current fields should be further explored
in the SHP at the local and landscape level.
38
CP1:
Weeping Lovegrass (WLG)
Old World Bluestem (OWB)
CP2:
Native w/ Buffalo Grass (NA-B)
Native w/o Buffalo Grass (NA-A)
Figure 2.1. Location of counties and study fields within the Texas Southern High Plains used in study, 2001 – 2003.
39
Table 2.1. Avian percent compositions and total numbers observed (in parenthesis) in CRP cover types for the breeding season (May – July)
2001 and 2002 in the Southern High Plains of Texas.
Cover TypeB
Species
CodeA
2001
WLG
2002
Total
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
NAA
2002
Total
Total #
Observed
(156)
(199)
(355)
(187)
(190)
(377)
(163)
(225)
(388)
(145)
(158)
(303)
GRSP
64.7
(101)
63.3
(126)
63.9
(227)
75.4
(141)
74.6
(141)
74.8
(282)
82.8
(135)
79.6
(179)
80.9
(314)
40.7
(59)
36.7
(58)
38.6
(117)
CASP
15.4
(24)
12.9
(23)
13.2
(47)
9.6
(18)
14.8
(28)
12.2
(46)
0
3.1
(7)
1.8
(7)
44.8
(65)
48.3
(77)
46.9
(142)
WEME
1.9
(3)
7.8
(14)
4.8
(17)
5.4
(10)
3.2
(6)
4.2
(16)
6.8
(11)
15.1
(34)
11.6
(45)
6.2
(9)
7.0
(11)
6.6
(20)
MODO
4.5
(7)
1.7
(23)
8.5
(30)
4.8
(9)
5.3
(10)
5.0
(19)
0
0.4
(1)
0.3
(1)
2.8
(4)
2.5
(4)
2.6
(8)
WEKI
7.1
(11)
4.5
(8)
5.4
(19)
1.6
(3)
0.5
(1)
1.1
(4)
1.2
(2)
1.8
(4)
1.5
(6)
1.4
(2)
0
0.7
(2)
LARB
3.9
(6)
0
1.7
(6)
2.7
(5)
0.5
(1)
1.6
(6)
4.3
(7)
0
1.8
(7)
0.7
(1)
3.8
(6)
2.3
(7)
RNPH
1.3
(2)
0.6
(1)
0.8
(3)
0.5
(1)
0
0.3
(1)
0.6
(1)
0
0.3
(1)
0.7
(1)
0.6
(1)
0.7
(2)
RWBL
1.3
(2)
0
0.6
(2)
0
0
0.3
(1)
0.61
(1)
0
0.3
(1)
0
0
0
BUOR
0
1.1
(2)
0.6
(2)
0
0
0
0
0
0
0
0
0
40
Table 2.1. Continued. Avian percent compositions and total numbers observed (in parenthesis) in CRP cover types for the breeding season
Cover TypeB
Species
CodeA
2001
WLG
2002
Total
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
STFC
0
1.1
(2)
0.6
(2)
MALL
0
0
GWTE
0
GTGR
NAA
2002
0
0.5
(1)
0.3
(1)
0
0
0
1.4
(2)
0
0.7
(2)
0
0
1.4
(1)
0.3
(1)
0
0
0
0
0
0
0
0
0
1.4
(1)
0.3
(1)
0
0
0
0
0
0
0
0
0
0
0
0
3.7
(6)
0
1.5
(6)
0
0
0
BUOW
0
0
0
0
0
0
0
0
0
0.7
(1)
0
0.3
(1)
BLGR
0
0
0
0
0
0
0
0
0
0.7
(1)
0
0.3
(1)
HOLA
0
0
0
0
0
0
0
0
0
0
0.6
(1)
0.3
(1)
Total
A
For species codes see Appendix A.
B
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss.
41
Table 2.2. Avian percent compositions and total numbers observed (in parenthesis) in CRP cover types for May 2001 and 2002 in the Southern
High Plains of Texas.
Cover TypeB
Species
CodeA
2001
WLG
2002
Total
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
NAA
2002
Total
Total #
Recorded
63
69
132
78
73
151
74
107
181
50
66
116
GRSP
55.6
(35)
68.1
(47)
62.1
(82)
73.1
(57)
83.6
(61)
78.1
(118)
83.8
(62)
74.8
(80)
78.5
(142)
42.0
(21)
33.3
(22)
37.1
(43)
CASP
19.1
(12)
14.5
(10)
16.7
(22)
6.4
(5)
11.0
(8)
8.6
(13)
0
4.7
(5)
2.8
(5)
48.0
(24)
42.4
(28)
44.8
(52)
WEME
3.17
(2)
8.7
(6)
6.1
(8)
7.7
(6)
1.4
(1)
4.6
(7)
2.7
(2)
19.6
(21)
12.7
(23)
8.0
(4)
7.6
(5)
7.8
(9)
MODO
3.17
(2)
2.9
(2)
3.0
(4)
5.1
(4)
1.4
(1)
3.3
(5)
0
0.9
(1)
0.6
(1)
0
3.0
(2)
1.7
(2)
WEKI
3.1
(2)
2.9
(2)
3.0
(4)
3.9
(3)
0
2.0
(3)
1.4
(1)
0
0.6
(1)
0
3.0
(2)
1.7
(2)
LARB
12.7
(8)
0
6.1
(8)
3.9
(3)
0
2.0
(3)
4.1
(3)
0
1.7
(3)
0
9.1
(6)
5.2
(6)
RNPH
1.6
(1)
0
0.8
(1)
0
0
0
0
0
0
0
0
0
0
42
Table 2.2. Continued. Avian percent compositions and total numbers observed (in parenthesis) in CRP cover types for May 2001 and 2002 inhe.
Cover TypeB
Species
CodeA
2001
WLG
2002
Total
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
NAA
2002
Total
RWBL
1.6
(1)
1.5
(1)
1.5
(2)
0
0
0
0
0
0
0
0
0
BUOR
0
1.5
(1)
0.8
(1)
0
0
0
0
0
0
0
0
0
MALL
0
0
0
0
1.4
(1)
0.7
(1)
0
0
0
0
0
0
GWTE
0
0
0
0
1.4
(1)
0.7
(1)
0
0
0
0
0
0
GTGR
0
0
0
0
0
0
8.1
(6)
0
3.3
(6)
0
0
0
BLGR
0
0
0
0
0
0
0
0
0
2
(1)
0
1.7
(1)
HOLA
0
0
0
0
0
0
0
0
0
0
1.5
(1)
1.7
(1)
A
For species codes see Appendix A.
B
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss.
43
Table 2.3. Total number and average number of avian species in CRP fields based on cover type and CP (conservation practice) type during the
breeding season (May – July) and May 2001, 2002, and years combined in the Southern High Plains of Texas
Cover Type A
CP Type A, B
WLG
OWB
NAB
NAA
CP1
CP2
Breeding Season Totals:
Total Numbers of Species (n)
2001
8
7
7
10
8
12
2002
8
8
5
7
10
8
Years Combined
10
10
9
11
10
13
2001
3.44 + 0.62
2.89 + 0.40
2.08 + 0.34
2.58 + 0.55
3.17 + 0.35
2.33 + 0.31
2002
3.78 + 0.62
3.44 + 0.22
2.50 + 0.22
2.33 + 0.24
3.61 + 0.30
2.42 + 0.15
Years Combined
3.61 + 0.40
3.17 + 0.24
2.29 + 0.20
2.46 + 0.28
3.39 + 0.23
2.38 + 0.17 *
May Totals:
Total Numbers of Species (n)
2001
8
6
5
4
8
7
2002
7
6
4
7
9
7
Years Combined
9
8
7
8
11
9
2001
3.33 + 0.88
3.33 + 0.88
3.00 + 0.71
2.75 + 0.48
3.33 + 0.56
2.88 + 0.40
2002
4.00 + 0.58
3.00 + 0.58
3.25 + 0.63
3.25 + 0.25
3.50 + 0.43
3.25 + 0.31
Years Combined
3.66 + 0.49
3.17 + 0.48
3.13 + 0.44
3.00 + 0.27
3.42 + 0.34
3.06 + 0.25
Avg. Species Number (n)
Avg. Species Number (n)
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
44
Table 2.4. Breeding season species abundance, richness, evenness, and dominance means (x̄) in CRP fields based on cover type and CP (conservation
practice) type for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA
CP TypeA
WLG
OWB
NAB
NAA
CP1
CP2
Species Abundance (n / ha)
2001
1.19 + 0.26
1.31 + 0.15
0.91 + 0.13
0.91 + 0.10
1.25 + 0.14
0.91 + 0.08
2002
1.26 + 0.39
1.33 + 0.20
1.29 + 0.09
1.04 + 0.26
1.29 + 0.14
1.17 + 0.14
Years Combined
1.22 + 0.15
1.32 + 0.11
1.10 + 0.10
0.98 + 0.13
1.27 + 0.09
1.04 + 0.08
2001
0.83 + 0.14
0.64 + 0.21
0.34 + 0.16
0.60 + 0.22
0.73 + 0.12
0.50 + 0.13
2002
0.80 + 0.15
0.76 + 0.06
0.51 + 0.09
0.56 + 0.11
0.78 + 0.07
0.54 + 0.07
Years Combined
0.81 + 0.09
0.70 + 0.10
0.45 + 0.09
0.58 + 0.12
0.76 + 0.07
0.52 + 0.07
2001
0.69 + 0.04
0.62 + 0.13
0.43 + 0.18
0.55 + 0.18
0.65 + 0.06
0.49 + 0.12
2002
0.61 + 0.04
0.62 + 0.04
0.59 + 0.10
0.60 + 0.09
0.61 + 0.04
0.60 + 0.06
Years Combined
0.65 + 0.03
0.62 + 0.06
0.51 + 0.10
0.60 + 0.09
0.63 + 0.03
0.54 + 0.07
2001
0.55 + 0.06
0.65 + 0.12
0.44 + 0.10
0.44 + 0.06
0.60 + 0.07
0.44 + 0.05
2002
0.58 + 0.07
0.59 + 0.04
0.71 + 0.06
0.50 + 0.06
0.59 + 0.04
0.60 + 0.06
Years Combined
0.57 + 0.04
0.62 + 0.06
0.57 + 0.07
0.47 + 0.04
0.59 + 0.04
0.52 + 0.04
Species Richness (H’)
Species Evenness (J)
Species Dominance (D)
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
45
Table 2.5. May species abundance, richness, evenness, and dominance means (x̄) in CRP fields based on cover type and CP (conservation practice)
types for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA
CP TypeA
WLG
OWB
NAB
NAA
CP1
CP2
Species Abundance (n / ha)
2001
1.34 + 0.25
1.58 + 0.15
1.38 + 0.16
0.96 + 0.16
1.45 + 0.14
1.17 + 0.13
2002
1.43 + 0.27
1.53 + 0.27
1.93 + 0.16
1.25 + 0.26
1.48 + 0.17
1.59 + 0.19
Years Combined
1.38 + 0.17
1.56 + 0.14
1.65 + 0.15
1.11 + 0.15
1.47 + 0.11
1.38 + 0.12
2001
0.87 + 0.21
0.70 + 0.28
0.60 + 0.24
0.60 + 0.15
0.79 + 0.16
0.60 + 0.13
2002
0.82 + 0.20
0.52 + 0.19
0.76 + 0.20
0.82 + 0.13
0.67 + 0.14
0.79 + 0.11
Years Combined
0.85 + 0.13
0.61 + 0.15
0.68 + 0.15
0.71 + 0.10
0.73 + 0.10
0.69 + 0.09
2001
0.76 + 0.05
0.57 + 0.10
0.46 + 0.17
0.61 + 0.10
0.67 + 0.07
0.53 + 0.09
2002
0.59 + 0.09
0.46 + 0.13
0.68 + 0.12
0.70 + 0.09
0.52 + 0.08
0.69 + 0.07
Years Combined
0.68 + 0.06
0.52 + 0.08
0.57 + 0.10
0.65 + 0.06
0.60 + 0.05
0.61 + 0.06
2001
0.50 + 0.10
0.65 + 0.13
0.43 + 0.16
0.74 + 0.05
0.57 + 0.08
0.58 + 0.10
2002
0.57 + 0.11
0.72 + 0.11
0.58 + 0.10
0.54 + 0.08
0.65 + 0.08
0.56 + 0.06
Years Combined
0.54 + 0.07
0.69 + 0.08
0.50 + 0.09
0.64 + 0.06
0.61 + 0.06
0.57 + 0.06
Species Richness (H’)
Species Evenness (J)
Species Dominance (D)
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
46
Table 2.6. Average breeding season (May – July) relative abundances for the dominate avian species in CRP fields based on cover type and
CP (conservation practice) type for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA
CRP TypeA
WLG
OWB
NAB
NAA
CP1
CP2
Grasshopper Sparrow (n / ha)
2001
0.78 + 0.11
0.99 + 0.22
0.74 + 0.14
0.38 + 0.22
0.88 + 0.12
0.56 + 0.14
2002
0.88 + 0.09
0.99 + 0.17
0.99 + 0.12
0.36 + 0.21
0.94 + 0.09
0.67 + 0.16
Years Combined
0.83 + 0.07
0.99 + 0.12
0.86 + 0.09
0.37 + 0.14
0.90 + 0.07
0.62 + 0.10
2001
0.18 + 0.05
0.13 + 0.11
0.00 + 0.00
0.38 + 0.13
0.15 + 0.05
0.19 + 0.09
2002
0.16 + 0.10
0.18 + 0.06
0.23 + 0.09
0.17 + 0.04
0.21 + 0.06
Cassin’s Sparrow (n / ha)
Years Combined
0.17 + 0.05
0.20 + 0.08
ab
0.16 + 0.06
0.05 + 0.04
b
0.02 + 0.02
0.42 + 0.13
b
0.40 + 0.08
a
Western Meadowlark (n / ha)
2001
0.02 + 0.01
0.07 + 0.01
0.06 + 0.02
0.06 + 0.03
0.05 + 0.01
0.06 + 0.02
2002
0.10 + 0.04
0.04 + 0.01
0.19 + 0.07
0.07 + 0.01
0.07 + 0.02
0.13 + 0.04
Years Combined
0.06 + 0.02
0.06 + 0.01
0.13 + 0.04
0.06 + 0.02
0.06 + 0.01
0.10 + 0.02
2001
0.06 + 0.02
0.06 + 0.01
0.00 + 0.00
0.03 + 0.03
0.06 + 0.01
0.02 + 0.02
2002
0.02 + 0.00
0.07 + 0.03
0.05 + 0.02
0.01 + 0.01
0.05 + 0.01
0.01 + 0.01 *
Mourning Dove (n / ha)
Years Combined
0.04 + 0.1
ab
0.07 + 0.01
0.01 + 0.02
a
0.01 + 0.01
0.02 + 0.04
b
0.03 + 0.02
ab
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= iIntroduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
47
Table 2.7. Average breeding season (May- July) singing male relative abundances for the dominate avian species in CRP fields based on
cover type and CP (conservation practice) type for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA
CRP TypeA, B
WLG
OWB
NAB
NAA
CP1
CP2
Grasshopper Sparrow (n / ha)
2001
0.38 + 0.04
0.55 + 0.16
0.36 + 0.09
0.19 + 0.11
0.47 + 0.08
0.28 + 0.08
2002
0.37 + 0.01
0.42 + 0.03
0.47 + 0.10
0.13 + 0.08
0.40 + 0.02
0.30 + 0.09
Years Combined
0.38 + 0.05
0.49 + 0.08
0.41 + 0.07
0.16 + 0.07
0.43 + 0.04
0.29 + 0.06
2001
0.10 + 0.0
0.04 + 0.03
0.00 + 0.00
0.27 + 0.08
0.07 + 0.02
0.14 + 0.06
2002
0.09 + 0.05
0.11 + 0.03
0.16 + 0.06
0.09 + 0.02
0.15 + 0.04
Cassin’s Sparrow (n / ha)
Years Combined
0.10 + 0.03
0.13 + 0.05
a
0.09 + 0.03
0.04 + 0.03
a
0.02 + 0.02
0.27 + 0.09
a
0.27 + 0.05
b
Western Meadowlark (n / ha)
2001
0.01 + 0.01
0.01 + 0.01
0.04 + 0.02
0.03 + 0.01
0.01 + 0.00
0.03 + 0.01
2002
0.02 + 0.02
0.01 + 0.01
0.06 + 0.04
0.01 + 0.01
0.01 + 0.01
0.03 + 0.02
Years Combined
0.01 + 0.01
0.01 + 0.00
0.05 + 0.10
0.02 + 0.01
0.01 + 0.01
0.03 + 0.01 *
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
48
Table 2.8. Average May relative abundances for the dominate avian species in CRP fields based on cover type and CP (conservation practice) type
for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA,C
WLG
OWB
NAB
NAA
CP1
CP2
Grasshopper Sparrow (n / ha)
2001
0.83 + 0.11
2002
0.99 + 0.08
Years Combined
0.91 + 0.07
1.20 + 0.07
1.02 + 0.19
1.28 + 0.29
ab
1.24 + 0.14
0.42 + 0.26
1.34 + 0.14
a
1.18 + 0.12
0.43 + 0.25
a
0.42 + 0.17
b
1.01 + 0.10
0.72 + 0.19
1.14+ 0.15
0.88 + 0.22
1.08 + 0.09
0.80 + 0.14
0.19 + 0.05
0.21 + 0.11
0.19 + 0.09
0.30 + 0.11
0.19 + 0.05
0.25 + 0.08
Cassin’s Sparrow (n / ha)
2001
0.27 + 0.06
2002
0.21 + 0.13
Years Combined
0.24 + 0.06
0.11 + 0.06
0.00 + 0.00
0.17 + 0.14
ab
0.14 + 0.07
0.42 + 0.17
0.12 + 0.12
b
0.06 + 0.06
0.47 + 0.14
b
0.45 + 0.10
a
Western Meadowlark (n / ha)
2001
0.04 + 0.04
0.13 + 0.07
0.03 + 0.03
0.08 + 0.01
0.08 + 0.04
0.06 + 0.02
2002
0.13 + 0.04
0.02 + 0.02
0.35 + 0.14
0.09 + 0.02
0.07 + 0.03
0.22 + 0.08
Years Combined
0.08 + 0.03
0.07 + 0.04
0.19 + 0..09
0.08 + 0.01
0.08 + 0.02
0.14 + 0.05
2001
0.04 + 0.04
0.08 + 0.04
0.00 + 0.00
0.00 + 0.00
0.06 + 0.03
0.00 + 0.00
2002
0.04 + 0.02
0.02 + 0.02
0.02 + 0.02
0.03 + 0.03
0.03 + 0.01
0.03 + 0.02
Years Combined
0.04 + 0.02
0.05 + 0.03
0.01 + 0.01
0.02 + 0.02
0.05 + 0.02
0.01 + 0.01 *
Mourning Dove (n / ha)
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
49
Table 2.9. Average May singing male relative abundances for the dominate avian species in CRP fields based on cover type and CP
(conservation practice) type for 2001, 2002, and years combined in the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA
WLG
OWB
NAB
NAA
CP1
CP2
Grasshopper Sparrow (n / ha)
2001
0.45 + 0.11
0.59 + 0.11
0.52 + 0.12
0.18 + 0.11
0.52 + 0.08
0.35 + 0.10
2002
0.52 + 0.06
0.59 + 0.06
0.61 + 0.11
0.21 + 0.12
0.56 + 0.04
0.41 + 0.11
Years Combined
0.49+ 0.06
0.59 + 0.06
0.57 + 0.08
0.20 + 0.08
0.54 + 0.04
0.38 + 0.07
2001
0.17 + 0.02
0.06 + 0.04
0.00 + 0.00
0.34 + 0.22
0.12 + 0.03
0.17 + 0.09
2002
0.13 + 0.10
0.13 + 0.07
0.24 + 0.08
0.12 + 0.04
0.20 + 0.06
Cassin’s Sparrow (n / ha)
Years Combined
0.15 + 0.05
0.13 + 0.13
ab
0.10 + 0.06
0.10 + 0.10
a
0.05 + 0.05
0.39 + 0.09
a
0.36 + 0.08
b
Western Meadowlark (n / ha)
2001
0.00 + 0.00
0.02 + 0.02
0.00 + 0.00
0.05 + 0.02
0.01 + 0.01
0.03 + 0.01
2002
0.04 + 0.04
0.00 + 0.00
0.11 + 0.09
0.00 + 0.00
0.02 + 0.02
0.06 + 0.05
Years Combined
0.02 + 0.02
0.01 + 0.01
0.06 + 0.05
0.03 + 0.01
0.02 + 0.01
0.04 + 0.02
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
50
Table 2.10. Summer (May – July) vegetative composition of CRP fields based on cover type and CP (conservation practice) type in the Southern
High Plains of Texas (2001 – 2002).
Cover Type A, B
CP Type A,C
WLG
OWB
NAB
NAA
CP1
CP2
Total plant species (n)
19
20
21
33
25
41
11.67 + 2.33
15.67 + 1.20
13.75 + 0.85
19.25 + 1.55
13.67 + 1.48
16.50 + 1.32
3
4
7
10
6
13
2.33 + 0.33 a
4.67 + 0.33 b
5.00 + 0.00 b
6.00 + 0.71 b
3.50 + 0.56
5.50 + 0.38 *
11
12
10
17
13
21
Avg. forb species/ field
7.33 + 1.20
8.67 + 1.20
7.50 + 0.96
9.00 + 1.35
8.00 + 0.82
8.25 + 0.82
Total shrub species (n)
5
4
4
6
6
7
3.00 + 1.00
2.67 + 0.33
1.75 + 0.25
2.25 + 0.85
2.83 + 0.48
2.00 + 0.42
Avg. plant species/ field
Total grass species (n)
Avg. grass species/ field
Total forb species (n)
Avg. shrub species/ field
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
51
Table 2.11. Dominant grass speciesA compositions and proportions among CRP cover typesB in the Southern High Plains of Texas (2001-2002).
WLG
Grasses:
Weeping lovegrass
Silver bluestem
Johnsongrass
Switchgrass
OWB
(98.2 + 0.8)
(0.7 + 0.5)
(0.3 + 0.2)
(0.3 + 0.2)
Old World bluestem
Blue grama
Johnsongrass
Sideoats grama
Switchgrass
NAB
(91.4 + 1.8)
(4.4 + 1.2)
(2.9 + 1.1)
(0.4 + 0.3)
(0.3 + 0.2)
Sideoats grama
Blue grama
Buffalograss
Japanese brome
Old World bluestem
Silver bluestem
Switchgrass
NAA
(75.2 + 2.0)
(11.4 + 1.4)
(5.1 + 1.0)
(3.6 + 0.8)
(3.2 + 0.9)
(0.4 + 0.4)
(0.2 + 0.2)
Sideoats grama
Switchgrass
Blue grama
Weeping lovegrass
Green sprangletop
Sand Dropseed
Johnsongrass
Silver bluestem
Arizona cottontop
Purple threeawn
Old World bluestem
Buffalograss
(32.6 + 2.3)
(16.0 + 1.7)
(11.8 + 1.5)
(11.2 + 1.6)
(5.5 + 1.0)
(5.3 + 1.1)
(3.9 + 1.1)
(3.8 + 1.1)
(3.3 + 0.3)
(3.2 + 0.9)
(0.5 + 0.4)
(0.4 + 0.2)
A
Scientific names are listed in Appendix
B
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss.
52
Table 2.12. Most common forb and shrub speciesA compositions among CRP cover typesB in the Southern High Plains of Texas (2001-2002).
WLG
OWB
NAB
NAA
Blueweed sunflower
Mare’s tail
Prickly lettuce
Mustard spp.
Ragweed
Fourwing saltbush
Silver-leaf nightshade
Goldenweed
Blueweed sunflower
Ragweed
Mare’s tail
Fourwing saltbush
Mustard spp
Curly-top gumweed
Rubber rabbitbush
Silver-leaf nightshade
Prickly lettuce
Rubber rabbitbush
Prickly lettuce
Purple ground cherry
Silver-leaf nightshade
Milkweed
Mare’s tail
Kochia
Mustard spp.
Mare’s tail
Common sunflower
Russian thistle
Silver-leaf nightshade
Mustard spp.
Curly-top gumweed
Plains ironweed
Goldenweed
Buffalo bur
Ragweed
Yucca
Catclaw
Baccharis
Yucca
Catclaw
Hackberry
Yucca
Baccharis
Hackberry
Yucca
Mesquite
Baccharis
Catclaw
Forbs:
Shrubs:
A
Scientific names are listed in Appendix
B
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss.
53
Table 2.13. Summer (May – July) vegetation characteristics of CRP fields based on cover type and CP (conservation practice) type in the Southern
High Plains of Texas (2001-2002)
Cover TypeA, B
CRP TypeA, C
Vegetation Characteristics
WLG
OWB
NAB
NAA
CP1
CP2
% Canopy Cover:
Grass
Living Grass
Dead Grass
76.42 + 2.31a
63.17 + 6.29a
77.56 + 1.85a
26.91 + 6.00b
69.79 + 4.21
52.23 + 10.00
40.97 + 3.84 a
62.54 + 6.54 ab
56.35 + 4.86 ab
65.89 + 4.15b
51.76 + 5.88
61.12 + 3.47
a
ab
ab
b
23.53 + 6.82
47.05 + 5.93
33.55 + 5.42
3.69 + 0.64
58.33 + 4.29
35.76 + 5.47
43.57 + 4.85
8.69 + 1.86 *
Forb
3.08 + 1.26
4.29 + 0.33
5.96 + 1.22
11.42 + 3.13
Total
79.50 + 1.88 a
67.46 + 6.42 a
83.52 + 2.92 a
38.32 + 5.35 b
73.48 + 4.03
60.92 + 8.99
b
a
13.44 + 1.29
11.71 + 4.02
1.01 + 0.18
0.69 + 0.07
13.10 + 4.69
27.39 + 7.67
% Litter
Litter Depth (cm)
14.43 + 2.37
a
1.15 + 0.38
12.46 + 1.32
ab
0.87 + 0.66
a
0.67 + 0.06
ab
0.74 + 0.14
a
Vertical Density (dm)
1.65 + 0.17 a
1.47 + 0.20 ac
0.79 + 0.11 bc
0.63 + 0.17 b
1.56 + 0.12
0.71 + 0.10 *
Avg. Vegetation Height (dm)
4.37 + 0.38 a
2.72 + 0.20 b
2.31 + 0.17 b
2.14 + 0.29 b
3.54 + 0.42
2.23 + 0.16 *
12.24 + 5.99 a
0.90 + 0.82 b
0.60 + 0.31 b
2.44 + 0.87 a
6.57 + 3.71
1.52 + 0.55
a
b
b
b
62.48 + 7.17
44.80 + 1.73 *
78.13 + 2.32
Max. Forb Height (cm)
29.60 + 5.06
46.83 + 2.61
37.76 + 2.87
a
Max. Living Height (cm)
75.14 + 3.46
Max. Dead Height (cm)
52.37 + 5.34 a
48.01 + 1.85
41.80 + 1.22
36.20 + 3.66
bc
40.31 + 2.35 ab
39.60 + 0.58
42.71 + 10.66
b
6.13 + 2.43
Max. Grass Height (cm)
12.07 + 2.17
19.01 + 6.25
% Bare Ground
Number Shrubs per 50 m
20.08 + 7.45
4.41 + 0.89
47.80 + 2.56
27.56 + 3.38
c
37.00 + 2.34 b
33.68 + 3.18
31.88 + 2.82
b
61.58 + 6.31
43.99 + 1.94 *
31.63 + 1.90 b
46.34 + 3.75
34.32 + 1.72 *
48.38 + 2.08
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
54
Table 2.14. Summer (May – July) vegetation characteristics of CRP fields based on cover type and CP (conservation practice) type for 2001 in
the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA, C
Vegetation Characteristics
WLG
OWB
NAB
NAA
CP1
CP2
% Canopy Cover:
Grass
Living Grass
Dead Grass
71.44 + 5.62 a
58.89 + 4.78 a
74.04 + 3.24 a
25.13 + 6.43 b
65.17 + 4.33
49.58 + 9.83
43.87 + 3.40
60.96 + 6.91
56.35 + 4.86
63.11 + 3.81
52.41 + 5.14
61.67 + 2.66
45.33 + 5.18
30.86 + 4.64
4.39 + 0.90
12.96 + 3.33 *
54.96 + 3.69
a
35.71 + 5.31
ab
36.95 + 3.84
ab
24.77 + 7.81
b
Forb
3.94 + 1.93
4.83 + 0.42
9.08 + 2.39
16.83 + 6.00
Total
75.39 + 4.04 ab
63.72 + 4.68 ab
83.13 + 4.20 a
41.96 + 7.89 b
69.56 + 3.80
62.54 + 8.81
b
a
19.00 + 2.73
13.27 + 3.73
24.19 + 7.13
% Litter
20.89 + 3.88
a
17.11 + 4.31
a
5.13 + 1.23
21.42 + 4.38
Litter Depth (cm)
% Bare Ground
3.72 + 1.77 a
19.17 + 8.28 ab
Vertical Density (dm)
1.57 + 0.31
1 .27 + 0.20
Avg. Vegetation Height (dm)
4.45 + 0.63 a
2.71+ 0.13 ab
11.75 + 3.08 a
36.63 + 11.17 b
11.44 + 5.12
0.91 + 0.18
0.67 + 0.30
1.42 + 0.18
0.79 + 0.17 *
2.40 + 0.30 ab
2.33 + 0.49 b
3.58 + 0.48
2.37 + 0.26 *
Number Shrubs per 50 m
Max. Grass Height (cm)
75.39 + 4.55 a
40.39 + 1.44 b
41.15 + 1.22 b
47.08 + 2.58 b
57.89 + 8.11
Max. Forb Height (cm)
32.71 + 5.29
42.29 + 3.61
38.29 + 2.67
31.40 + 6.96
37.51 + 3.58
Max. Living Height (cm)
Max. Dead Height (cm)
74.12 + 6.14
a
40.09 + 7.24
a
44.31 + 0.95
bc
37.92 + 3.10
ab
43.79 + 0.71
c
29.16 + 3.11
b
48.88 + 3.51
b
59.22 + 7.22
33.40 + 1.79
b
39.01 + 3.56
44.12 + 1.73
34.84 + 3.69
a
46.34 + 1.92
31.28 + 1.84
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
55
Table 2.15. Summer (May – July) vegetation characteristics of CRP fields based on cover type and CP (conservation practice) type for 2002 in
the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA, C
Vegetation Characteristics
WLG
OWB
NAB
NAA
CP1
CP2
% Canopy Cover:
Grass
Living Grass
Dead Grass
Forb
Total
81.39 + 1.50 a
67.44 + 7.99 a
81.08 + 2.20 a
28.69 + 5.68b
74.42 + 4.79
54.89 + 10.30
38.08 + 4.67 a
64.13 + 6.80b
49.58 + 6.11ab
71.55 + 4.19 b
51.10 + 6.89
60.57 + 5.39
a
b
48.76 + 6.91
36.23 + 6.59
2.99 + 0.51
4.42 + 1.05
77.40 + 4.73
59.30 + 9.54
7.89 + 0.92
10.15 + 4.74
1.01 + 0.18
0.69 + 0.07
30.59 + 8.53
61.71 + 5.04
a
2.22 + 0.76
83.61 + 2.18
% Litter
7.97 + 1.12
Litter Depth (cm)
1.15 + 0.38
35.81 + 6.75
ab
3.75 + 0.34
a
71.19 + 8.29
2.83 + 0.57
a
7.81 + 1.72
16.60 + 8.76
0.67 + 0.06
ab
0.74 + 0.14
a
b
48.79 + 10.73
14.76 + 4.36
8.53 + 3.39
Vertical Density (dm)
1.74 + 0.04 a
1.65 + 0..21 a
0.67 + 0.05 b
0.60 + 0.10 b
1.70 + 0.10
0.64 + 0.05 *
Avg. Vegetation Height (dm)
4.29 + 0.19 a
2.72 + 0.30b
2.23 + 0.06 b
1.95 + 0.17 b
3.51 + 0.39
2.09 + 0.10 *
12.24 + 5.99 a
0.90 + 0.82 b
0.60 + 0.31 b
2.44 + 0.87 a
6.57 + 3.71
1.52 + 0.55
a
b
b
b
67.07 + 6.42
45.48 + 1.94*
Max. Grass Height (cm)
80.88 + 0.17
Max. Forb Height (cm)
26.48 + 6.03
53.27 + 3.90
33.23 + 2.19
a
Max. Living Height (cm)
76.16 + 1.52
Max. Dead Height (cm)
64.64 + 4.15 a
51.71 + 2.75
12.40 + 1.89
34.69 + 4.01
b
% Bare Ground
Number Shrubs per 50 m
21.00 + 6.67
83.92 + 2.18
22.28 + 5.97
6.00 + 1.77
a
3.69 + 0.55
0.87 + 0.66
a
50.19 + 6.08
42.45 + 1.27
34.10 + 4.79
b
42.69 + 2.08 b
35.42 + 0.94
48.51 + 3.14
23.73 + 2.20
c
44.85 + 2.63 b
29.85 + 3.24
28.91 + 3.13
b
63.93 + 5.64
41.65 + 2.73*
29.86 + 5.00 b
53.67 + 5.33
37.35 + 3.86*
47.88 + 2.84
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
56
Table 2.16. Correlation coefficientsA between bird abundancesB and vegetation variables in CRP fields during the 2001 breeding season in the Southern
High Plains of Texas.
Total Abundance
% Veg.
Cover
% Native
Grass
Cover
% Living
Grass
Cover
% Litter
Cover
% Bare
Ground
Cover
Vertical
Density
Shrub
Density
Max.
Grass
Height
0.0051
-0.3711
-0.1157
0.3213
-0.1712
0.2278
0.4768
0.1114
0.5199
0.0129
-0.0940
0.0810
-0.6067 *
0.5776 *
0.0440
0.0401
*
*
-0.0307
-0.0123
-0.4028
0.6256 *
0.4473
*
0.4698
Grasshopper Sparrow
Total
Singing Male
0.4354
-0.0347
-0.0474
0.1636
-0.5575
0.5394
Total
-0.8025 ***
-0.3955
-0.0787
0.4653 *
0.4068
Singing Male
***
-0.4205
-0.1101
0.5462
*
0.3754
-0.3960
06526
-0.2146
0.2562 *
0.6748 **
-0.1527
0.5664
-0.3610 *
-0.3178
-0.5509
-0.3687
0.5778
*
**
0.0507
0.4654
-0.6447
*
-0.0427
-0.3479
-0.2417
-0.7096 **
0.5375 *
-0.0962
0.1856
0.4159
0.0915
Cassin’s Sparrow
-0.7980
Western Meadowlark
Total
Singing Male
0.5069
Mourning Dove
Total
A *
B
-0.1103
P < 0.05, ** P < 0.01, *** P < 0.001
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment correlation coefficients. All other
species-specific values are Spearman rank correlation coefficients.
57
Table 2.17. Correlation coefficients A between bird abundancesB and vegetation variables in CRP fields during the 2002 breeding season in the Southern
High Plains of Texas.
% Vegetation
Cover
% Dead
Grass
Cover
% Forb
Cover
% Litter
Cover
% Bare
Ground
Cover
Vertical
Density
Avg. (x̄)
Vegetation
Height
-0.7465 **
-0.6519 *
0.5617 *
0.1821
0.7284 **
-0.8583 ***
-0.8510 ***
Total
0.7674 **
0.5559 *
-0.6453 *
-0.1144
-0.5807 *
0.4118
0.3837
Singing Male
**
*
-0.5279
-0.2698
-0.5335
*
03216
0.3483
Total Abundance
Grasshopper Sparrow
-0.7665
0.5878
Cassin’s Sparrow
Total
-0.6497 *
-0.5812
0.3827
0.4436
0.6298 *
-0.2718
-0.4442
Singing Male
-0.7271 **
-0.6563 *
0.3584
0.4701
0.6983 **
-0.3315
-0.5304
Total
0.2873
0.3094
-0.4226
-0.4314
-0.2829
-0.3514
0.0111
Singing Male
0.3705
0.2462
-0.1171
-0.5099
-0.2681
-0.2998
0.0171
0.0440
-0.0857
-0.0626
0.2086
-0.0579
0.7363 **
0.4052
Western Meadowlark
Mourning Dove
Total
58
Table 2.17. Continued. Correlation coefficients between bird abundances A and vegetation variables in CRP fields during the 2002 breeding
Max.
Grass
Height
Max.
Forb
Height
Max.
Dead
Height
Total
Plant
Species
Grass
Species
Forb
Species
-0.6873 **
-0.1794
-0.7989**
0.7251 **
0.8195 ***
0.4438
Total
0.1230
0.6224 *
0.5615 *
-0.3092
-0.2470
-0.7280
Singing Male
0.0477
0.6881 **
0.5678 *
-0.3142
-0.2549
-0.6927 **
Total
0.1635
-0.2829
-0.5060
0.0579
0.1879
0.3277
Singing Male
0.0597
-0.3227
-0.6188
0.1370
0.2801
0.3669
Total
-0.3956
0.2674
0.2541
0.0657
-0.1069
0.0828
Singing Male
-0.2949
0.5118
0.1974
0.1032
-0.1327
0.2578
0.4793
-0.1459
0.1088
-0.3348
-0.3689
Total Abundance
Grasshopper Sparrow
**
Cassin’s Sparrow
Western Meadowlark
Mourning Dove
Total
A *
B
-0.3738
P < 0.05, ** P < 0.01, *** P < 0.001
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment correlation coefficients. All other
species-specific values are Spearman rank correlation coefficients.
59
Table 2.18. Correlation coefficients A between bird abundances B and vegetation variables in CRP fields during the breeding season (2001 – 2002) in the
Southern High Plains of Texas.
% Grass
Cover
% Native
Grass
Cover
% Bare
Ground
Cover
Vertical
Density
Max.
Forb
Height
Max.
Living
Height
Max.
Dead
Height
Shrub
Density
0.4346
0.1067
-0.1361
0.2573
0.5852 *
0.0866
0.3843
0.1060
Total
0.7997 ***
0.2938
-0.5807 *
0.4503
0.6224 *
0.0241
0.5615 *
-0.2118
Singing Male
***
0.4124
-0.5335
*
0.3519
0.6881
*
-0.0513
0.5678
*
-0.2565
-0.5696 **
0.0783
0.5632 **
-0.3148
-0.4418 *
0.3126
-0.2143
0.5291 **
-0.6209 ***
-0.2884
0.5841 **
-0.3523
-0.4655 *
0.2524
-0.2164
0.5022 **
0.1184
0.2445
0.2785
-0.3164
-0.0466
-0.5136 **
0.1743
-0.1433
-0.1473
-0.0064
0.3257
0.2217
Total Abundance
Grasshopper Sparrow
0.7785
Cassin’s Sparrow
Total
Singing Male
Western Meadowlark
Total
Singing Male
*
-0.0012
0.4388
-0.0382
-0.6414 ***
0.0943
-0.4271
*
0.1974
-0.3854
-0.0596
0.4537 *
-0.0203
0.3269
*
Mourning Dove
Total
A *
B
P < 0.05, ** P < 0.01, *** P < 0.001
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment correlation coefficients. All other
species-specific values are Spearman rank correlation coefficients.
60
Table 2.19. Correlation coefficientsA between bird abundancesB and vegetation variables in CRP fields during May 2001 in the Southern High Plains of
Texas.
% Grass
Cover
% Native
Grasses
Cover
% Vegetation
Cover
% Bare
Ground
Cover
Vertical
Density
Shrub
Density
Max.
Grass
Height
Grass
Species
(n)
0.4540
0.0715
0.3786
-0.3676
0.2558
-0.0345
-0.2304
-0.2842
Total
0.6217 *
0.1987
0.6329 *
-0.6440 *
0.5572 *
-0.4607
-0.2462
-0.1103
Singing Male
*
0.2050
0.5823
*
-0.6319
*
0.5044
-0.4101
-0.1628
-0.1304
Total
-0.4672
-0.4917
-0.6176 *
0.3500
-0.4392
0.7271 **
0.6451 *
0.0141
Singing Male
-0.5211
-0.4665
-0.6512 *
0.3898
-0.4242
0.6852 **
0.6054 *
0.0759
-0.1172
0.2605
-0.1400
-0.0115
0.1125
0.0115
-0.1033
0.2349
0.3576
-0.1663
-0.0486
-0.0554
0.5687 *
-0.0429
0.1257
0.3191
-0.1778
-0.2902
Total Abundance
Grasshopper Sparrow
0.5858
Cassin’s Sparrow
Western Meadowlark
Total
Singing Male
-0.6272
*
0.0101
-0.6404
-0.5359 *
-0.1042
*
Mourning Dove
Total
A *
B
-0.0245
P < 0.05, ** P < 0.01
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment correlation coefficients. All other
species-specific values are Spearman rank correlation coefficients.
61
Table 2.20. Correlation coefficientsA between bird abundancesB and vegetation variables in CRP fields during May 2002 in the Southern High Plains of
Texas.
% Dead
Grass
Cover
% Vegetation
Cover
% Litter
Cover
% Bare
Ground
Cover
Vertical
Density
Avg. (x̄)
Vegetation
Ht.
Max.
Grass
Height
Max.
Forb
Height
-0.6907 **
-0.5739 *
0.2628
0.4302
-0.6806 *
-0.6950 *
-0.7027 *
-0.2469
Total
0.5263
0.7231 **
-0.2516
-0.7439 *
0.5273
0.4574
-0.0394
0.6511 *
Singing Male
0.6253 *
0.7385 **
-0.2700
-0.7673 **
0.4875
0.5258
0.0786
0.5216
-0.5632 *
-0.6383 *
0.4454
0.4731
-0.3112
-0.4922
-0.0743
-0.3492
-0.4798
-0.6312
-0.1586
-0.3976
-0.4587
-0.1232
0.2196
0.1275
0.4609
-0.4740
0.0214
0.4015
0.1254
0.6269 *
-0.1984
0.3096
0.0930
0.4664
-0.4149
Total Abundance
Grasshopper
Sparrow
Cassin’s Sparrow
Total
Singing Male
-0.6755
**
-0.5792
*
0.4314
0.5332
*
Western Meadowlark
Total
Singing Male
0.2640
0.4805
0.3396
0.5725
0.1623
0.1832
-0.3920
*
-0.5847
*
Mourning Dove
Total
0.0026
62
Table 2.20. Continued.
Maximum
Dead
Height
Total
Plant
Species
Grass
Species
Forb
Species
-0.8023 **
0.6506 *
0.5722 *
0.4871
Total
0.3969
-0.1426
-0.1303
-0.5851 *
Singing Male
0.4834
-0.3071
-0.2457
-0.7071 **
Total
-0.3965
0.0942
0.1468
0.4231
Singing Male
-0.4827
0.1598
0.2188
0.4956
Total
0.2260
0.1590
-0.0159
0.0283
Singing Male
0.2966
0.1079
-0.1308
0.0464
Total
0.0902
-0.3038
-0.2912
-0.1918
Total Avian abundance
Grasshopper Sparrow
Cassin’s Sparrow
Western Meadowlark
Mourning Dove
A *
P < 0.05, ** P < 0.01
B
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment
correlation coefficients. All other species-specific values are Spearman rank correlation coefficients.
63
Table 2.21. Correlation coefficients A between bird abundancesB and vegetation variables in CRP fields during May (2001 – 2002) in the Southern High
Plains of Texas.
% Grass
Cover
% Native
Grass
Cover
% Litter
Cover
% Bare
Ground
Cover
Vertical
Density
Avg. (x̄)
Vegetation
Height
Max.
Forb
Height
Shrub
Density
0.5358 *
0.5344 *
-0.2553
-0.2615
0.2265
0.3220
0.6058 *
-0.2273
Total
0.7420 **
0.4304
-0.2516
-0.5512 *
0.5765 *
0.5423 *
0.5808 *
-0.4379
Singing Male
**
0.3820
-0.2700
-0.5718
*
*
*
0.4271
-0.3948
Total
-0.5174 **
-0.3572
0.4200 *
0.4567 *
-0.4165 *
-0.3582
-0.3726 *
0.5193 **
Singing Male
-0.5801 **
-0.3053
0.4027 *
0.5164 **
-0.4982 **
-0.4234 *
-0.3779 *
0.5093 **
0.2518
0.1648
-0.2112
-0.1160
-0.1041
0.0888
0.3447
0.0354
Total Abundance
Grasshopper Sparrow
0.7577
0.5476
0.5966
Cassin’s Sparrow
Western Meadowlark
Total
Singing Male
*
-0.0362
0.2736
0.0653
0.1815
-0.1151
-0.0528
-0.0512
0.2460
0.4743
0.1694
-0.3403
-0.0210
-0.0797
0.2196
0.1153
-0.1062
Mourning Dove
Total
A *
B
P < 0.05, ** P < 0.01, *** P < 0.001
Values given for Total Abundance and Grasshopper Sparrow abundances are Pearson product-moment correlation coefficients. All other
species-specific values are Spearman rank correlation coefficients.
64
Table 2.22. Avian percent compositions and total numbers observed (in parenthesis) in CRP cover types for the wintering season (January – February)
2002 and 2003 in the Southern High Plains of Texas.
Cover TypeB
Species
CodeA
2001
WLG
2002
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
NAA
2002
Total
Total #
Observed
(27)
(40)
(24)
(45)
(69)
(112)
(118)
(230)
(64)
(173)
(237)
WEME
63.0
(17)
62.5
(25)
62.7
(42)
33.3
(8)
62.2
(28)
52.2
(36)
25.0
(28)
44.1
(52)
34.8
(80)
31.3
(20)
26.0
(45)
27.4
(65)
SAPS
18.5
(5)
35.0
(14)
28.4
(19)
8.3
(2)
20.0
(9)
15.9
(11)
21.4
(24)
16.1
(19)
18.7
(43)
34.4
(22)
31.2
(54)
32.1
(76)
GRSP
0
0
0
4.2
(1)
0
1.4
(1)
2.7
(3)
0
1.3
(3)
0
0
0
VESP
0
0
0
0
0
0
0
0.8
(1)
0.4
(1)
0
0
0
HOLA
0
0
0
50.0
(12)
13.3
(6)
26.1
(18)
41.1
(46)
39.0
(46)
40.0
(92)
31.3
(20)
38.2
(66)
36.3
(86)
RNPH
11.1
(3)
2.5
(1)
6.0
(4)
4.2
(1)
2.2
(1)
1.4
(1)
0
0
0
0
0
0
Total
(67)
65
Table 2.22. Continued. Avian percent compositions and total numbers observed in CRP Cover Types for the wintering season (January – February).
Cover TypeB
Species
CodeA
2001
WLG
2002
Total
2001
OWB
2002
Total
2001
NAB
2002
Total
2001
NAA
2002
Total
NOHA
3.7
(1)
0
1.5
(1)
0
0
0
0.9
(1)
0
0.4
(1)
0
1.7
(3)
1.3
(3)
PRFA
3.7
(1)
0
1.5
(1)
0
0
0
0
0
0
0
0
0
SEOW
0
0
0
0
2.2
(1)
1.4
(1)
3.6
(4)
0
1.7
(4)
0
0
0
BUOW
0
0
0
0
0.5
(1)
1.4
(1)
3.6
(4)
0
1.7
(4)
3.1
(2)
1.7
(3)
2.1
(5)
GTT0
0
0
0
0
0
0
0
0
0
0
0.6
(1)
0.4
(1)
0
0
0
0
1.8
(2)
0
0.9
(2)
0
0.6
(1)
0.4
(1)
MODO
0
0
0
0
A
For species codes see Appendix A.
B
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss.
66
Table 2.23. Wintering season species abundance, richness, evenness, and dominance means (x̄) in CRP fields based on cover type and CP (conservation
practice) type for 2002, 2003, and years combined in the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA, C
WLG
OWB
NAB
NAA
CP1
CP2
Average Number of Species
2002
3.00 + 1.53
3.00 + 1.00
5.00 + 0.00
3.00 + 0.41
3.00 + 0.82
4.00 + 0.42
2003
Years Combined
2.00 + 1.15
2.50 + 0.89
2.67 + 0.33
a
3.75 + 0.48
2.83 + 0..48
a
4.38 + 0.32
5.00 + 1.00
b
4.00 + 0.63
b
2.33 + 0.56
4.38 + 0.56
2.67 + 0.48
4.19 + 0.34 *
0.57 + 0.16
1.08 + 0.08
0.61 + 0.13
1.09 + 0.09
0.59 + 0.100
1.08 + 0.06 *
0.46 + 0.12
0.82 + 0.05
0.52 + 0.12
0.80 + 0.05
0.49 + 0.08
0.81 + 0.03
Species Richness (H’)
2002
0.47 + 0.26
2003
0.48 + 0.27
Years Combined
0.47 + 0.17
0.68 + 0.23
1.18 + 0.10
0.74 + 0.03
a
0.71 + 0.10
0.97 + 0.11
1.14 + 0.13
ab
1.16 + 0.08
1.04 + 0.14
c
1.01 + 0.08
bc
Species Evenness (J)
2002
0.41 + 0.21
2003
0.46 + 0.23
Years Combined
0.44 + 0.14
0.51 + 0.14
0.73 + 0.06
0.57 + 0.13
a
0.54 + 0.09
0.92 + 0.01
0.88 + 0.03
a
0.80 + 0.04
0.73 + 0.07
b
0.82 + 0.05
b
Species Dominance (D)
2002
0.66 + 0.20
0.69 + 0.13
0.41 + 0.06
0.41 + 0.04
0.68 + 0.11
0.41 + 0.03
2003
0.38 + 0.20
0.53 + 0.02
0.37 + 0.04
0.45 + 0.04
0.46 + 0.10
0.41 + 0.03
Years Combined
0.52 + 0.14
0.61 + 0.07
0.39 + 0.03
0.43 + 0.03
0.57 + 0.08
0.41 + 0.02
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
67
Table 2.24. Average wintering season (January – February) relative abundances for the dominate avian species in CRP fields based on cover
type and CP (conservation practice) type for 2002, 2003, and years combined in the Southern High Plains of Texas.
Cover TypeA, B
CRP TypeA, C
WLG
OWB
NAB
NAA
CP1
CP2
Western Meadowlark (n / ha)
2001
0.89 + 0.14
0.42 + 0.14
1.09 + 0.38
0.78 + 0.11
0.65 + 0.14
0.94 + 0.19
2002
1.25 + 0.72
1.45 + 0.52
1.95 + 0.75
1.75 + 0.45
1.35 + 0.40
1.86 + 0.41
Years Combined
1.07 + 0.34
0.94 + 0.34
1.52 + 0.42
1.27 + 0.28
1.00 + 0.23
1.40 + 0.25
Savannah Sparrow (n / ha)
2001
0.26 + 0.26
0.10 + 0.05
0.94 + 0.37
0.86 + 0.33
0.18 + 0.12
0.90 + 0.23
2002
0.73 + 0.58
0.47 + 0.39
0.74 + 0.26
2.11 + 0.49
0.60 + 0.32
1.43 + 0.36
Years Combined
0.49 + 0.30
0.29 + 0.20
0.84 + 0.21
1.48 + 0.36
0.39 + 0.78
1.16 + 0.22 *
2001
0.00 + 0.00
0.52 + 0.52
1.75 + 0.81
0.70 + 0.48
0.26 + 0.26
1.23 + 0.48
2002
0.00 + 0.00
0.31 + 0.31
Horned Lark (n / ha)
Years Combined
1.72 + 0.54
a
1.73 + 0.45
2.46 + 1.95
b
1.58 + 0.99
0.16 + 0.16
2.09 + 0.95
ab
0.21 + 0.15
1.66 + 0.53 *
0.00 + 0.00
0.42 + 0.28
2001
1.41 + 0.55 a
1.20 + 0.66 a
4.22 + 0.52 b
2.42 + 0.66 ab
1.30 + 0.39
3.32 + 0.52
2002
2.08 + 1.38
2.34 + 0.72
4.61 + 1.32
6.64 + 0.2.26
2.21 + 0.70
5.63 + 1.27
1.74 + 0.68 a
1.77 + 0.51 a
4.41 + 0.66 b
4.53 + 1.35 b
1.76 + 0.40
4.47 + 0.73 *
Total Abundance (n / ha)
Years Combined
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
68
Table 2.25. Winter vegetation characteristics of CRP fields based on cover type and CP conservation type (CP) in the Southern High Plains of
Texas for 2002.
Cover TypeA
CRP TypeB
Vegetation Characteristics
WLG
OWB
NAB
NAA
CP1
CP2
% Canopy Cover:
Grass
57.33 + 4.81 a
68.89 + 5.58 a
78.50 + 3.54 a
26.29 + 5.78 b
63.11 + 4.21
52.40 + 10.35
Forb
1.78 + 0.71
1.17 + 0.29
% Litter
28.67+ 2.87 ad
Litter Depth (cm)
0.77 + 0.11
% Bare Ground
15.89 + 3.32 bcd
8.42 + 0.99 bc
0.94 + 0.43
12.22 + 7.68
0.42 + 0.11
14.06 + 8.35
a
Vertical Density (dm)
1.67 + 0.19
Avg. Vegetation Height (dm)
3.36 + 0.41 a
Number Shrubs per 50 m
2.04 + 0.99
10.89 + 6.63
a
Max. Grass Height (cm)
69.09 + 8.09
Max. Forb Height (cm)
52.44 + 12.49 a
1.14 + 0.09
11.04 + 4.53
ac
0.68 + 0.05
3.96 + 0.48
1.47 + 0.37
3.00 + 0.63
36.25 + 7.17 a
22.27 + 3.47
22.33 + 6.24
0.61 + 0.21
0.86 + 0.20
0.51 + 0.11
13.14 + 5.09
22.35 + 6.34
33.67 + 9.05
bc
0.35 + 0.16
b
1.40 + 0.15
0.51 + 0.10 *
2.34 + 0.17 ac
2.02 + 0.12 bc
1.32 + 0.36 bc
2.85 + 0.30
1.67 + 0.22 *
0.89 + 0.86
0.47 + 0.23
2.85 + 2.07
45.62 + 5.33
bc
47.63 + 3.06
29.48 + 0.44 a
ab
33.70 + 4.09 a
5.88 + 3.73
1.66 + 1.06
b
57.36 + 6.80
43.55 + 2.80
27.40 + 3.56 b
40.96 + 7.59
30.55 + 2.78
39.47 + 4.02
A
WLG = weeping love grass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
CP1= introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
C
CP type means ( + SE ) with an (*) superscript are different (P > 0.05).
69
Table 2.26. Correlation coefficientsA between 2002 winter bird abundancesB and vegetation variables from Winter 2002 and Summer (July) 2001
in CRP fields in the Southern High Plains of Texas.
Winter
Litter
Depth
Winter
Avg. Veg.
Height
Forb
Species
Summer
% Litter
Cover
Summer
Vertical
Density
Summer
Avg. Veg.
Height
Total Avian Abundance
-0.6058 *
-0.3032
-0.1094
-0.7122 **
-0.5304
-0.5146
Western Meadowlark
-0.0268
-0.1697
0.6396 *
-0.3103
-0.2210
0.0692
Savannah Sparrow
-0.2749
-0.4716
0.4163
-0.3934
-0.3866
-0.3263
Horned Lark
-0.5752 *
-0.6239 *
-0.0715
-0.3900
-0.7214 **
-0.7604 **
A*
B
P < 0.05, ** P < 0.01
Values given for total avian abundance are Pearson product-moment coefficients. All species values are Spearman rank
correlation coefficients.
70
Table 2.27. Correlation coefficientsA between 2003 winter bird abundancesB and vegetation characteristics from Summer (July) 2002 in CRP fields in
the Southern High Plains of Texas.
Forb
Species
% Forb
Cover
% Vegetation
Cover
%Bare
Ground
Cover
Vertical
Density
Avg.
Vegetation
Height
Max.
Forb
Height
Total Avian Abundance
0.4384
0.0249
-0.1094
0.1219
-0.5066
-0.4911
0.1389
Western Meadowlark
0.3293
0.0829
0.6396 *
-0.0285
-0.1299
-0.2398
07658 *
Savannah Sparrow
0.7077 **
0.5431 *
0.4163
0.5736 *
-0.4688
-0.2985
-0.2017
Horned Lark
0.5332 *
-0.1313
0.0160
0.0160
-0.6207 *
-0.7443 **
0.2084
A*
B
P < 0.05, ** P < 0.01
Values given for total avian abundance are Pearson product-moment coefficients. All species values are Spearman rank
correlation coefficients.
71
CHAPTER III
NESTING BIOLOGY AND NEST SITE CHARACTERISTICS OF GROUNDNESTING BIRDS IN THE PRINCIPLE GRASSLAND COVER TYPES
IN THE SOUTHERN HIGH PLAINS OF TEXAS
Introduction
According to the North American Breeding Bird Survey (BBS), grassland birds
have declined substantially over the past 25 years (Peterjohn and Sauer 1993, Askins
1993). The loss, fragmentation, and degradation of the historical grassland habitat that
these birds rely on have all been cited as primary factors for these declines (Peterjohn and
Sauer 1993, Herkert 1994, Knopf 1994, Herkert et al. 1996, Johnson 2001). Of the
approximate 162 million ha of native prairie that once existed before settlement by
Europeans, less than 90% remains today (Samson and Knopf 1994). Loss of native
grasslands has resulted in obligate grassland birds (i.e., birds that rely on these habitats
for all or portions of their life history) having to find other alternatives that are available
and can be substituted for historic natural habitats within their geographic ranges.
Grassland birds, in the absence of native vegetation, will breed in grassland habitats that
are structurally similar to those historically used (Bollinger et al. 1990, Warner 1994,
Koford 1999). The quality of this habitat and its suitability in providing nesting cover and
food resources, however, are often the main limiting factors in birds producing a
successful nest and, thus, in maintaining populations.
Before the Conservation Reserve Program (CRP) was implemented under the
Federal Food Security Act of 1985, grassland birds in the absence of historical nesting
habitat would often nest in low quality prairie remnants, non-native pastureland and
hayfields, or in other types of non-native habitat such as cropland residues. With the
intensification of farming practices in the late 1950s through the mid 1980s the situation
for grassland nesting birds became considerably worse (Best et al. 1997, Brady 2000).
From 1966 to 1996, 44% of the grassland nesting birds had a declining trend reflecting
the impact that the loss of quality nesting habitat was having on this avian group (Brady
72
2000). From 1980 through 2002, which includes the 17 years of the CRP fields being
available for breeding birds, the number of species with declining population trends did
not change (15 species out of 25, or 60%)(Peterjohn and Sauer1999). However, between
1966 and 1980 10 species were record as having significantly negative population trends,
while between 1980 and 2002, that number dropped to only 5 (Peterjohn and Sauer
1999).
It was only after the CRP program was implemented that reversals in these trends
were first observed. CRP enrollment currently stands at 13.5 million ha, with
approximately 80% of CRP acres planted with grass cover (Heard 2000). Since its
initiation there have been numerous studies that have documented the positive effects of
the CRP for grassland birds. Johnson and Schwartz (1993a) observed that several
grassland bird species (e.g., Grasshopper Sparrow (Ammodramus savannarum) and Lark
Bunting (Calamospiza melanocorys)) in the Northern Great Plains that had seriously
declined in abundance during 1966-1990 were common in CRP fields. In the Midwest,
Best et al. (1997) found nests of 33 species in CRP fields compared with only 10 in rowcrop fields, and the number of nests found was 13.5 times greater in CRP fields. Patterson
and Best (1996) concluded that the CRP has likely contributed to an increase in the
abundance of many bird species in central Iowa, inasmuch as the row-crop habitat that it
replaced had lower bird abundance and supported fewer nesting species. In the Southern
High Plains, Berthelsen et al. (1989) concluded that Ring-necked Pheasant populations
would benefit from CRP fields due to increases in nesting cover, brood cover, and winter
habitat. In the same study Berthelsen and Smith (1995) concluded that CRP was
providing quality breeding habitat for nongame birds, because it replaced acres that had
previously been cropland with nesting cover. McCoy et al. (1999) reported that the
Conservation Reserve Program can be useful in reducing threats to species population
declines and in maintaining stable populations of some avian species, but not others.
In an attempt to increase grassland habitat, CRP participants with existing
contracts in Texas (those from 1987 through 1993) have been required to re-seed 51% of
CRP land with native grasses since 1997. Furthermore, all new CRP contracts require
73
native species seeding of at least 90% of the area. However, the value of native seeded
CRP land in providing quality nesting habitat for obligate grassland birds has not been
assessed fully in the Southern High Plains. Despite the fact that Texas has more land
enrolled in CRP (1.6 million ha or 11.4% of the total as of 1997) than any other state,
only one study (Berthelsen 1989) has looked at the impact that it has had on nesting
grassland birds. To more fully understand the contributions and values of the different
seeding mixtures for grassland bird conservation in Texas, I investigated whether avian
densities and nesting success differed among the principle CRP cover types and, where
possible, native rangeland. Additionally, I examined what nest site vegetation
characteristics were associated with the different species, and if there were any
differences between successful and depredated nests, and between CP conservation type
fields for each species.
Methods
Study Area
I studied the nesting ecology of the ground-nesting grassland birds (> 5 nests)
that were located on five principle cover types (4 types of CRP and native short-grass
prairie) found within the Southern High Plains of Texas. This portion of the High Plains,
also known as the Llano Estacado, straddles the Texas-New Mexico border and is
bounded by the Canadian River to the North and the Caprock Escarpment to the South
and East (Blackstock 1979). In Texas, this region is characterized by nearly flat to gently
undulating featureless plains that include numerous natural depressions known as playa
lakes (Blakely and Koos 1974, Blackstock 1979). Study fields were selected from the
south-central and south-western sections of the SHP within Hale, Lamb, Lubbock, Terry
and Bailey counties (Figure 3.1). Elevation within these counties ranges from 880 meters
in the east to 1200 meters in the west.
The area has a dry continental-steppe climate with mild winters and most rainfall
events occurring from April to October (Blakely and Koos 1974, Blackstock 1979).
Normal average temperature during summer is 24.5 o C with the hottest temperatures
74
being recorded in July and August (26.6 o C and 25.6 o C, respectively) (National Oceanic
and Atmospheric Administration [NOAA] 2003). Maximum highs during this period
often exceed 31.5 o C (NOAA 2003). Annual average precipitation and temperature
fluctuate widely ranging from 22.1 cm to 102.9 cm, and -4o to above 50o C, respectively
(Blakely and Koos 1974, Blackstock 1979, Haukos and Smith 1997). The annual
variability in rainfall and temperature makes drought a frequent occurrence (Holliday
1991). Historically this area was dominated by short- to mixed-grass prairie with large
portions covered in buffalograss and blue grama. Today the landscape is dominated by
agriculture with large portions of the study area counties now being used for cotton, grain
sorghum, wheat, corn, and other grain crop production.
Field Selection
CRP study fields were chosen based on the original seed mixture, the degree and
quality of stand establishment, and recommendations from local USDA Natural
Resources Conservation Service (NRCS) personnel. Fields selected were a representative
sample from the larger population of CRP fields of good quality and degree of
establishment within in the SHP. The grass mixes chosen were those most representative
in the south-central and south-western portion of the SHP. These included 2 types of
introduced grasses (CP1s) and two types of native grass mix (CP2s). The CP1s were
monocultures of either ‘Old World’ bluestem (OWB) (Bothriochlora ischaemum) or
weeping lovegrass (WLG) (Eragrostis curvula), while the CP2s were a mix of 3 to 5
native grass species. The principle grasses include in native mixes were sideoats grama
(Bouteloua curtipendula), blue grama, green sprangletop (Leptochloa dubia), switchgrass
(Panicum virgatum), and buffalograss. Native mixes were differentiated into two cover
types based on whether buffalo grass was included (NAB) or absent (NAA) from the
native mix. Buffalo grass presence or absence in the native seed mix was selected as a
treatment because of its historical importance and abundance in native shortgrass prairies
(Berthelsen et al.1989), and because it is not always selected in native mixes due to its
cost and difficulty in getting established.
75
Fourteen CRP study fields were selected for study: 3 WLG, 3 OWB, 4 NAB, and
4 NAA. All fields were approximately 65 ha in area and separated by more than 0.5 km.
In addition to the CRP fields, 1 study field in 2001 and 2 study fields in 2002 were
selected from Muleshoe National Wildlife Refuge (MNWR) to represent native
shortgrass prairie. These fields had low densities of mesquite and had not been grazed
since 1997. Due to lack of adequate native prairie that met these criteria we did not
include them in the analyses of nesting densities, and only report means as a reference.
Study Plots and Nest Search Areas
A nest search area (NSA) of 4 ha (200 m x 200 m) was randomly chosen from the
4 quadrants of the 30.25 ha (550 m x 550 m) fixed-circular point count 3-x-3 grid
established in each field (see Chapter II). The corners of each NSA were the
corresponding centers of 4 of the 9-point count stations. Each NSA was searched twice
during the nesting period. In 2001 NSAs were searched from the 16 May–10 June and 17
June–10 July, and in 2002 from 12 May–2 June and 12 June–10 July. The same NSAs
within each study field were used in both years of the study.
Nest Searching and Monitoring
The search-line method was used to search NSAs for nests of ground-nesting
birds (Baxter and Wolfe 1973, Berthelsen et al. 1989). Fields were searched
systematically by teams of 2-3 individuals that would walk abreast (<1 m spacing) and
part the grass with sticks to get a view of the ground (Berthelsen et al. 1989). Alignment
and completeness of the search was maintained by the use of metal flags that were
removed and reset after each pass until the entire NSA was searched. NSAs were
searched twice during the breeding season, the timing corresponding with and following
the first and second avian abundance surveys (Chapter II). In 2001 and 2002, NSAs were
searched during the second and third week of May, and during the second and third
weeks of June. NSAs were surveyed from 0900 to 1400. NSAs were not searched during
76
periods of precipitation or if winds exceeded 18 km/hour to reduce the chances of
missing nests due to weather conditions.
Nests were located by flushing adult birds off nest and by observing adult
behavior. Behavioral clues that were used to identify nest locations included chipping,
flying short distances away from or around observers, flushing close to observers and
flying only a short distance, and the carrying of nest material, fecal sacs, or food (Winter
1999). Additional nests were located by observing adult behavior within the entire study
plot and fortuitously while collecting other data (i.e., point counts, vegetation sampling,
monitoring of nests).
Nest monitoring and recording protocol followed that outlined by Martin and
Geupel (1993) and Ralph et al (1993). Nest locations were marked with a metal flag
positioned 5 m N of the actual nest location. Nests were monitored every 2-3 day until an
outcome could be determined. Nests were considered successful if > 1 nestling fledged
per nest. The following clues based on Winter (1999) were used to determine if empty
nests were successful: (1) feces in nest, (2) feather sheaths in the nest, (3) nest rim
flattened, (4) adults carrying food or chipping, and/ or (5) fledgling close to nest.
Nest Success
Species daily nest survival rate was calculated for the incubation, nestling, and
total nesting period stages using the method developed by Mayfield (1975). Estimates of
incubation and nestling stage duration needed for calculations of nesting success were
taken from Ehrlich et al. (1988) for the Grasshopper Sparrow (20 days; 11 incubation
days plus 9 nestling days) and Mourning Dove (27 days; 14 incubation days plus 13
nestling days), and from Schnase et al. (1991) and Dunning (2000) for the Cassin’s
Sparrow (20 days; 11 incubation days plus 9 nestling days). Standard errors for daily nest
survival rates were calculated following Murphy et al. (1999). Means and confidence
intervals for incubation, nestling, and the total nesting period were calculated using the
method described by Johnson (1979). This method allows nest success comparisons
between years and treatments for all species by comparing confidence intervals. .
77
Nest Site Vegetation Characteristics
Nest vegetation was measured within 7 days of the nesting outcome. All
vegetation measurements were centered on or taken from the nest cup or depression. A
50 cm x 50 cm Daubenmire frame was used to estimate the percent grass, forb, litter, and
bare ground cover surrounding the nest on a non-overlapping basis (Daubenmire 1959).
Litter depth was measured at the mid-point of each side of the Daubenmire frame and
averaged to produce 1 mean per nest. Litter was defined as any dead or decaying
unattached vegetation lying flat on the soil surface. Maximum grass, forb, living, and
dead heights of vegetation with their bases inside the daubenmire frame were also
recorded at each nest. The vertical density and average height of the vegetation around
the nest was determined by placing a Robel pole (Robel 1970) at the entrance of a nesting
cup or in the middle of a nest depression depending on the species. Vertical density was
recorded as the lowest obstructed dm measured at 4m from the Robel pole and at a height
on 1 m above the ground in each of the four cardinal directions (Robel 1970). Vegetation
touching or within 2 cm of the Roble pole in each of the four cardinal directions was
averaged to determine the average vegetation height at each nest. The number of shrubs
within 50-m of the nest was counted to determine shrub densities around each nest, and
the distance from the nest to the nearest shrub was measured as the nearest perching site.
Statistical Analyses
Only those nests located inside the nest search areas (NSAs) were used in
estimating nest density for species, cover types, and CP types. The Shapiro-Wilk
Normality test (Shapiro and Wilk 1965) and Levene’s test (1960) were used to test the
assumptions of normality and the equality of variances among treatments, respectively.
The test of the assumptions indicated that the data for Cassin’s Sparrow densities were
non-normal and heterogeneous resulting in the non-parametric testing of this data. To test
for differences in total avian nest densities, and nest densities of the Grasshopper Sparrow
and Mourning Dove among the cover types and between CP type I used a repeated
measure ANOVA. The whole plot factor was cover type or CP type (independent
78
variables) and year was the repeated measure. If significant, this was followed by
Tukey’s HSD Test to separate the differences among the means (P < 0.05). I tested for
differences in Cassin’s Sparrow nest density among the cover types and betweenCP types
for 2001, 2002, and the years combined using Kruskal-Wallis one-way analysis and the
Mann-Whitney U Test, respectively (Conover 1980).
Differences in nest vegetation characteristics among species were tested using
one-way ANOVAs with measured vegetation variables as the dependent variable and
cover type as the independent variable. Tukey’s HSD Test was used to identify the
differences among the means if the ANOVA was significant (P < 0.05). Due to small
sample sizes of nests of individual species by cover type, I pooled nests of each species
by CP type to test for differences in nest site characteristics within introduced and native
grass mix CRP fields. A two-tailed t-test followed by a sequential Bonferroni adjustment
(P < 0.05) was used to compare the vegetation characteristics of Grasshopper Sparrow
and Mourning Dove nests in CP1 and CP2 study plots (Sokal and Rohlf 1995). Measured
vegetation variables were the dependent variables and CP type was the independent
variables. Cassin’s Sparrow was the only species with enough nests in native shortgrass
prairie plots to allow for comparisons with CP types. To compare vegetation
characteristics of Cassin’s Sparrow in CP1, CP2, and native shortgrass prairie a one-way
ANOVA was used followed by Tukey’s HSD Test if significant (P < 0.05).
Results
Nesting Composition and Densities
Seven species were found nesting within study plots, and 6 within NSAs (Tables
3.1 and 3.2). Cover types and CP types were similar in nesting species compositions and
total number of nests located (Tables 3.1 and 3.2). A total of 118 nests were located for
the 2 years combined. Nests of Grasshopper Sparrows (29.7%), Cassin’s Sparrows
(23.7%), and Mourning Doves (38.1%) accounted for 91.5% of all nests located. NSAs
within the CRP study plots yielded 69 nest, or 58.5% of the total; 28 Grasshopper
Sparrow, 10 Cassin’s Sparrow, and 22 Mourning Dove.
79
Total nest densities were similar among CRP cover types (F3,10 = 0.43, P =
0.7334), between CP types (F1,12 = 0.84, P = 0.3781), and between years (F1,10 = 0.43, P
= 0.2308 and F1,12 = 0.27, P = 0.2671), and there was no significant cover type x year
(F3,10 = 1.48, P = 0.2777) or CP type x year interaction (F1,12 = 0.03, P = 0.8707) (Table
3.3). Grasshopper Sparrow nest densities differed among CRP cover types (F3,10 = 2.99,
P = 0.0483), but when pooled by CP type they did not differ (F1,12 = 0.41, P = 0.5120).
Densities differed among native grass mixes with NAB nest search areas having more
nests per ha than NAA (P = 0.0434). There were no differences detected between
introduced grasses (P = 0.7925) or between the introduced grasses and NAB nest search
areas for the Grasshopper Sparrow (P = 0.1277 and P = 0.2001, respectively for WLG
and OWB). Cassin’s Sparrows were not found nesting in OWB or NAB in 2001 and in
NAB in 2002 (Table 3.3). Densities for this species did not differ in 2001, 2002, or years
combined among the remaining CRP cover type (H = 2.18, P = 0.5366; H = 2.05, P =
0.5612; H = 2.16, P = 0.5401, respectively), or between CP types (U = 23.5, P = 0.9157;
U = 20.0, P = 0.6056l; U = 21.0, P = 0.6985, respectively). Cassin’s Sparrow nest
densities were 3 times greater on native shortgrass prairie compared to the CRP cover
types, but, due to lack of replication, the means are only presented here as a reference of
possible, and apparent, biological significance. Grasshopper Sparrows and Mourning
Doves were not found nesting in either year within the NSA of the native shortgrass
prairie. Mourning Dove nest densities did not differ by CRP cover type (F3,10 = 1.63, P =
0.2435), CP type (F1,12 = 2.96, P = 0.1111), or years (F1,10 = 0.31, P = 0.5922 and F1,12 =
0.36, P = 0.5573), and there was no significant cover type x year (F3,10 = 0.13, P =
0.9410)or CP type x year interaction (F1,12 = 0.36, P = 0.5573) (Table 3.3).
Species Nesting Biology
The peak-breeding season for birds on the Southern High Plains is early to midMay with numbers tapering off through July (Berthelsen 1989). For determination of
approximate initiation dates, the breeding season was divided into three 28-day periods
and the number of nests identified through back dating to have been initiated during that
80
period were summed (Figure 3.2). For years combined, 76% of all nests were initiated
before 10 June and 96% were initiated by 7 July. Grasshopper Sparrows had 89% of their
nest initiated before 10 June and were not found nesting on any of the study plots after 8
July. Cassin’s Sparrows were also not found initiating nests within the study fields after
the 8th of July. The peak of nesting for Cassin’s Sparrow was during the first period of the
breeding season (71% of nest initiated), but nest initiations continued through the end of
the June (29% initiated). Mourning Doves initiated nests through the end of July but the
majority of nesting took place during the first (64%) and second (27%) periods of the
breeding season. Clutch size, hatching success, young-fledged per successful nest, and
nest productivity did not differ between years for any of the species (Table 3.4).
Nest Success
Nest success during incubation and nestling stages did not differ between years
for any species (Table 3.5). Nesting failure tended to occur more frequently during the
nestling stage compared to the incubation stage for both the Grasshopper Sparrow and
Cassin’s Sparrow (Table 3.5). In contrast, nesting failure occurred more frequently during
the incubation stage for the Mourning Dove.
Due to small sample sizes in both 2001 and 2002, I did not attempt to evaluate
yearly differences in nest success of individual species by cover type. Estimates of
species nest success by cover type and CP type are presented here with years combined
(Table 3.6). For all species nest success did not differ by CRP cover type or CP type; the
95% confidence intervals overlapped in all cases for all species. Grasshopper Sparrow
nest success tended to be higher on NAB study plots (60.7%) than the other CRP cover
types (39.4-29.3%). Nest success was also higher on CP2 study plots (55.7%) compared
to CP1 study plots (31.3%) for this species. Mayfield nest success estimates for the
Cassin’s Sparrow were similar among the CRP cover types in which it occurred most
often, WLG (41.6%) and NAA (40.1%), but tended to be higher on native shortgrass
prairie (50.6%). Between CP types, Cassin’s Sparrows had higher nest success in CP1s
(61.3%) than CP2s (40.1%). Mourning Dove nest success estimates were higher on WLG
81
study plots (41.3%) than the other CRP cover types (25.9%-20.5%). When the CRP cover
types were pooled, nest success was higher on CP1 study plots (35.4%) compared to CP2
study plots (20.5%).
Nest depredation was the main cause of nest failure for all species (Table 3.6). For
most nests the depredating species was unknown, although skunks (Mephitis mephitis.),
ground-squirrels (Citellus spp.), coyotes (Canis latrans) and several species of snakes
were seen on the study fields. Brown-headed Cowbirds (Molothrus ater) were observed
only once in the two years of this study in close proximity to CRP cover types. During
this study no nest located in CRP fields were parasitised, but in 2002 a Cassin’s Sparrow
nest in native shortgrass prairie field was abandoned after being parasitiesed by a Brownheaded Cowbird.
Nest Site Characteristics Between Species
The Grasshopper Sparrow, Cassin’s Sparrow, and Mourning Dove differed in nest
placement, nest plant, and nest site characteristics (Table 3.7). The Cassin’s Sparrow was
the only species that nested both above and at ground level. Sixteen of 28 Cassin’s
Sparrow nests (57.1%) were in a plant ranging from 1.0 to 8.4 cm above the ground (x̄ =
4.04 + 0.89). Cassin’s Sparrow nests were normally placed near the center of the nest
plant and woven into and around the stems for support. Within the CRP cover types
Cassin’s Sparrows primarily nested in weeping lovegrass (52.9%), switchgrass (17.6%),
‘Old World’ bluestem (17.6%), and sideoats grama (11.8%). Nests found in the native
shortgrass prairie were placed in broom snakeweed (Gutierrezia sarothraie) 72.7% of the
time, with the remaining nests being composed of blue grama, burrograss, and prairie
three-awn (Aristida purpurea).
Grasshopper Sparrow nests were found at ground level with the nest being
tunneled into and woven near the base of the nest plant. The main nest plants used were
sideoats grama-blue grama (42.9%), ‘Old World’ bluestem (22.9%), and weeping
lovegrass (20.0%). Grasshopper Sparrow used broom snakeweed as the nest plant in the
two nest found within the native shortgrass prairie study plots. Mourning Dove nests
82
were located near the base of vegetation or clumps of vegetation with adequate litter
cover and vegetation height suitable for the nest. Nests were not woven into the
vegetation but were mainly made up of litter arranged into a shallow bowl. Nests were
found 64.4% of the time under the introduced grass species (weeping love grass 40.0%,
‘Old World’ bluestem 24.4%). The remaining nest were predominately found under
clumps of sideoats grama and switch grass.
Of the 12 vegetation variables measured at each nest site only 4 were similar
among the three species (Table 3.7). The Grasshopper Sparrow, Cassin’s Sparrow, and
Mourning Dove did not differ in percent bare ground cover (F2,103 = 2.0, P = 0.1335),
vertical density (F2,105 = 1.70.0643), average vegetation height (F2,105 = 3.1, P = 0.0509),
or maximum forb height (F2,79 = 2.6, P = 0.0780). The Mourning Dove differed from the
other two species in having more litter cover (F2,103 = 11.5, P < 0.0001), greater litter
depth (F2,103 = 10.9, P < 0.0001), and higher living vegetation height (F2,105 = 11.1, P <
0.0001). The two sparrows differed from each other in the amount of grass (F2,105 = 4.9,
P = 0.0089) and forb (F2,103 = 7.1, P = 0.013) cover, height of dead vegetation (F2,105 =
5.3, P = 0.0069), shrub densities (F2,105 = 19.3, P < 0.0001), and nearest perch sites (F2,46
= 6.1, P < 0.0044). Grasshopper Sparrow nest had more grass and less forb cover, lower
dead vegetation height, and less shrub densities within 50 m of the nest. Cassin’s Sparrow
nests had less grass cover, more forb cover, higher dead vegetation, less distance to the
nearest perch site, and greater shrub densities within 50 m of the nest.
Species Nest-Site Characteristics between CP Types
Nest site characteristics were not analyzed at the specific cover type level or
between years because of small and unequal sample sizes within groups. For the
Grasshopper Sparrow, Cassin’s Sparrow, and Mourning Dove I compared nest
characteristics between the introduced grass study plots (CP1’s) and the native grass mix
study plots (CP2s). Additionally, I compared nest site characteristics between successful
and depredated nests, and between nests within CP1 and CP2 fields for all of these
83
species. Because I found a similar number of Cassin’s sparrow nests in native shortgrass
prairie and the individual CP types, I included range in the analyses.
Of the 12 vegetation variables measured at Grasshopper Sparrow nests, only
maximum living vegetation height was different between the two CPs (t31 = -3.9, P =
0.0004) (Table 3.8). CP1s had higher living vegetation heights than CP2 fields. No other
nest site characteristic differed between nests found in CP1 and CP2 study fields for the
two years. Depredated and successful nests differed in vertical density and average
vegetation height, both being higher on depredated nests (t31 = 2.4, P = 0.0251, and t31 =
2.3, P = 0.0299) (Table 3.9). However, these differences did not remain significant after
the Bonferroni adjustment. Successful and depredated nests within CP type did not differ
statistically in any of the measured vegetation variables at the nest site (Table 3.10).
Cassin’s Sparrow nests differed considerably between the CPs and native
shortgrass prairie (Table 3.11). Nests found on the native shortgrass prairie study plots
differed from the CP types in the percent forb cover (F2,25 = 19.7, P < 0.0001), vertical
density (F2,25 = 12.1, P = 0.0002), maximum living vegetation height (F2,25 = 8.5, P =
0.0016), and shurb density within 50 m of the nest site (F2,25 = 35.5, P < 0.001). CRP
study fields had less forb cover, higher vertical densities, higher living vegetation heights,
and less shrub densities than native rangeland study plots. Additionally, nest sites in CP2
study fields had higher percent bare ground (F2,25 = 12.3, P = 0.0002), greater distances
to the nearest perch sites (F2,22 = 9.1, P = 0.0014), and substantially lower shrub densities
(F2,25 = 35.5, P < 0.0001) than did nest sites in native rangeland. The three cover types
did not differ in percent litter cover or in litter depth (F2,25 = 2.8, P = 0.0816, and F2,25 =
0.5, P = 0.6199, respectively). Only 2 nest variables differed between nests found on CP1
and CP2 study plots; CP2 nests had less percent grass cover (F2,25 = 13.3, P = 0.001) and
more bare ground cover (F2,25 = 12.3, P = 0.0002) than CP1 nests. Cassin’s Sparrow
nests did not differ in any nest site characteristic between successful and depredated nests
(Table 3.12). Likewise, no differences were detected between successful and depredated
nests within CP types and native shortgrass prairie. However, before the Bonferroni
adjustment, successful nests in CP2 fields had lower maximum forb heights (t5 = 3.3, P =
84
0.0206) and were further away from the nearest perch sites (t3 = -3.5, P = 0.0390) (Table
3.13).
Eight of the 12 nest site variables differed between the CP types for Mourning
Dove, but only 4 remained significant after Bonferroni adjustments (Table 3.14). Nest
sites measured in CP1 study fields had higher percent grass cover (t41 = 4.24, P = 0.0001)
and lower percent forb (t41 = -2.47, P = 0.0177) and litter cover (t41 = -2.11, P = 0.0406)
than CP2 study fields. Vertical density (t41 = 3.82, P = 0.0004), average height of
vegetation (t41 = 3.89, P = 0.0004), maximum height of living and dead vegetation (t41 =
4.28, P = 0.0001, and t41 = 2.98, P = 0.0048, respectively), and shurb densities within 50
m of the nest (t41 = 2.24, P = 0.0302) were all higher on nest sites located in CP1 study
fields. Percent grass cover, vertical density, average vegetation height, and maximum
height of living vegetation were the only variables that remained significant after the
Bonferroni adjustment. No differences were detected in nest site characteristics between
successful and depredated nests for all Mourning Dove nests (Table 3.15), or for nests
within specific CP types (Table 3.16).
Discussion
In the Southern High Plains of Texas specific CRP cover types did not differ in
the number of nesting species, but they did differ in the composition and proportion of
nests found per species, presumably as a result of differences in habitat structure. The
Grasshopper Sparrow (32.0%), Cassin’s Sparrow (16.5%), and Mourning Dove (41.7%)
accounted for 90.1% of all nest found in CRP study fields. The number of nests found,
species compositions, proportions of nests per CRP cover type, and nest densities in this
study differed considerably from those reported by Berthelsen (1989) and Berthelsen and
Smith (1995). During this study I found less than a quarter (24.2%; n = 103) of the nests
found in their study (n = 425) during the late 1980s even with two additional study fields.
In Berthesen’s study the dominant species were the Ring-necked Pheasant (43.5%), Redwinged Blackbird (18.3%), Grasshopper Sparrow (15.3%), Cassin’s Sparrow (8.7%), and
Western Meadowlark (5.9%). In this study I only recorded 2 Ring-necked Pheasant nests,
4 Western Meadowlark nests, and found no Red-winged Blackbird nests. Part of this
85
difference is due to Berthelsen’s study taking place in counties located in the central and
west-central portion of the SHP within CRP fields that were relatively young stands (< 3
years). Differences in Ring-necked Pheasant nests can be attributed to the present study
taking place on the southwestern edge of its range in Texas (Sauer et al. 1996).
Additionally, the extreme variability within rainfall and temperatures within and between
years for the 2 studies, as well as differences in vegetation structure and landscape
attributes, probably also played a significant role in the differences observed (Rottenberry
and Wiens 1991).
Nest densities were considerably less than those reported by Berthelsen and Smith
(1995) for total avian species, Grasshopper Sparrow, Cassin’s Sparrow, and Western
Meadowlark nest densities. Berthelsen and Smith (1995) were finding approximately 3.4
nests per ha compared to 0.58 in this study. However, to the Berthelsen and Smith (1995)
study similar to their study relationships between species nest densities and cover types
were difficult to distinguish due to low samples sizes within the nest search areas. Total
avian densities did not differ between the cover types or CP types during this study. Nest
densities were similar between years for all species within the cover types and CP types.
This might reflect the influence that drought conditions over the past 5 years might have
on nest densities, causing species to nest in lower densities within the SHP or breeding in
other areas. Grasshopper Sparrow was the only species whose nest densities differed
among the 4 cover types, being more abundant in native mix with buffalo grass (NAB)
than in native mix without buffalo grass (NAA). Although Berthesen et al. (1989) did not
find any differences in Grasshopper Sparrow abundances among the 1 native mix and the
2 native/introduced grass mix, in both years the blue grama-sideoats grama mixes had
higher or equal nest densities compared to the other mixes. The differences in nest
densities among the cover types in this study are the probable result of differences in
vegetative structure among the cover types. NAB fields in this study had higher grass
cover, lower grass heights and vertical densities, and less litter cover than the other three
cover types. Cody (1968) and Wiens (1969 and 1973) both reported Grasshopper
Sparrows preferring to nest in areas of low to sparse vegetation with no or little shrub or
86
woody species. Wiens (1969) also reported that Grasshopper Sparrow territories had
lower vertical densities, forb and litter cover, and higher grass cover than areas outside
territories. Bollinger (1995) and Rottenberry and Weins (1980) reported Grasshopper
Sparrows abundances correlated with increasing grass cover. When cover types were
pooled into CPs nest densities did not differ for this species, reflecting the variablility
within cover types of the introduced and native grass mixes.
Cassin’s Sparrows were not found nesting on any NAB study field and were only
found to nest in 1 OWB field in 2002. The remaining two cover types, NAA and WLG,
were similar in nesting density but were approximately 3 times lower compared to native
shortgrass prairie. Berthelsen and Smith (1995) reported higher densities of nest on the
native grass mix compared to the native/exotic grass mixes, but densities were only
significantly higher in 2001. Previous studies have reported that Cassin’s Sparrow
territories normally require a shrub component with enough grass cover to meet the
species nesting requirements (Maxwell 1979, Dunning 1999, Ruth 2000). Males of this
species perform flight songs from available perches within their territory. These flight
songs have been hypothesized as serving the multiple functions of territorial
establishment, mate attraction, territory defense, and predator detection (Anderson and
Conway 2000). Ruth (2000) characterized Cassin’s Sparrow territories as ranging along
a continuum of varying shrub densities. Areas that lack or have too few shrubs, as well as
areas with too high of densities and not enough grass cover will usually not have Cassin’s
Sparrows, while habitats in between these 2 extremes are likely to meet the species
requirements. WLG fields had the highest shrub densities (x̄ = 12.24 + 5.99) among cover
types, while NAA fields had low to moderate shrub densities (x̄ = 244 + 0.87) but the
most bare ground of the cover types (x̄ = 42.71 + 10.66). In NAA fields where shrub
densities were < 2 per 50 m2, this species was observed to initiate their flight song from
areas of bare ground, and occasionally from the dead stems located at the tops of bunches
of ‘Old World’ bluestem and weeping love grass. This suggests that the Cassin’s
Sparrows might nests in areas with shrub densities below those reported from other
studies if they can substitute bare ground or other available vegetation as song and flight
87
song perches. However, based on available literature, nest densities would probably be
very low in these areas and would thus not serve as viable habitat for this species (i.e.,
population sink). While the native shortgrass prairie nests were not included in the
analyses with the CRP cover types, for both 2001 and 2002 3-times as many Cassin’s
Sparrow nest were found in these areas. Native prairie within this study had moderate
densities of yucca, and low densities of mesquite, as well as a grass and forb component
for nesting substrates that might better met the territorial requirements of nesting Cassin’s
Sparrows compared to CRP fields.
Mourning Dove nest densities were relatively similar among the cover types, but
on the average higher on the introduced grass fields. In contrast to this study, Berthelsen
and Smith (1995) did not report the nest densities of this species, presumably due to low
sample sizes, and possibly reflecting the nesting habitat adaptability of this species (i.e.,
able to nest in areas other than CRP fields) (Sayre and Silvy 1993). Within this study
cover types were characterized by tall clumped vegetation with moderate levels of bare
ground and litter cover. George et al. (1979) and Houghes (2000) also noted Mourning
Doves nesting in these types of fields. Mourning Doves were able to nest near the bases
of these clumps that would shield nests from direct sunlight, while the available litter and
bare ground underneath these clumps provided material and area for the placement of the
nest.
Clutch sizes, hatching success, and nest productivity did not differ between years
for the Grasshopper Sparrow, Cassin’s Sparrow, or Mourning Dove, but it was slightly
less than that reported by Berthelsen (1989). We found no statistical differences in nest
success between the cover types or CP types among the species in this study when the
years were combined based on calculated confidence intervals from Johnson (1979). This
suggests that cover types within our study area are relatively equivocal in terms of nest
success. However, lack of sufficient sample sizes within the cover types might be
occluding any differences in nest success being detected as was noted in the results.
Granfors et al. (1996), Koford (1999), and McCoy et al. (2001) were also unable to detect
any differences in nest success due to cover type (CRP fields and native range; CRP
88
fields and waterfowl production areas; and CP1 and CP2 CRP fields). Total nest success
for Grasshopper Sparrow and Cassin’s Sparrow were similar to slightly higher to those
reported by Berthelsen and Smith (1995), and is on the high end of the reported 25-50%
average for grassland-nesting birds (Weins 1968, Martin 1995, Greenwood et al.1995).
Mourning Dove apparent nest success was similar to the 55.6% reported by Houghes et
al. (2000) on CRP in eastern Kansas, but considerably higher than studies in Kansas
(29%; Downing 1959) and Texas (16%; Swank 1955). Nest depredation was the main
cause of nest failures for all species.
Nest placement and nest site characteristics differed considerably between
species, as would be expected. Likewise, nests of specific species differed in vegetation
characteristics depending upon the CP type, suggesting that within the CPs species were
able to use and select from a wide continuum of vegetation variables or alternatives to
these. Of the 3 main species in this study only the Cassin’s Sparrow is recognized as a
primary species (endemic) to grasslands (Knopf 1994). The Grasshopper Sparrow is
recognized as a secondary, more widespread species (Knopf 1994), while the Mourning
Dove is adaptable to nesting in a variety of habitats, not just grasslands (Sayre and Silvy
1993). Cassin’s Sparrows and Grasshopper Sparrows were only occasionally found to
nest in the same cover types and never within great proximity to each other. As
mentioned above the Cassin’s sparrow will normally not nest in habitats lacking adequate
shrub densities for perching and sky-larking and grass cover for nesting (Maxwell 1979,
Dunning 1999, Ruth 2000), while Grasshopper Sparrows tended to avoid areas with
increasing shrub and forb densities, preferring areas with increasing grass cover (Weins
1969, Rottenberry and Wiens 1980, Bollinger 1995). Within CP types no differences
were detected between any of the measured nest site vegetation characteristics and the
fate of the nest, suggesting that species were able to select areas within the CP types that
met nesting requirements for that species. However, a larger sample size and further
information about reproductive productivity would be needed to discern the varying
qualities of the different nest sites for individual species within cover types and CP types
and between nest fates within these plantings.
89
Management Implications
The native seeding requirement of CRP in the SHP has benefited nesting
grassland birds by providing large areas of established grassland that vary in vegetative
composition and structure in which to breed. Individual species demonstrated a
preference for specific cover types and vegetative characteristics related to the nest site.
Management within these cover types should focus on recognizing the breeding
requirements of species of management concern and manipulating the structure of CRP
fields through the use of management practices that simulate natural disturbances (e.g.,
plantings, grazing, fire, discing) and successional stages.
During this study, the pooling of cover types into the broad categories of CP1s
(exotic grasses) and CP2s (native grass mixes) often tended to dampen or occlude
biologically meaningful results. For this reason the broad categories based solely on CP
alone should be avoided when assessing the value of different CRP cover types.
Vegetation structure and vegetative composition in so far as helps to determine this
structure likely influences how grassland birds utilize CRP fields during the breeding and
wintering season. Native grass mixes and the individual grass species in these mixes,
with their differences in germination rate, growth form, production, successional
development, and intra- and interspecific interactions with other plant species, are more
likely to provide and meet the requirement of more grassland birds due to the variability
created in vegetative composition and structure than monocultures of exotic grasses.
The relationships between species nest densities and cover types, and species nest
success and cover types were difficult to distinguish due to low samples sizes within
treatments. Larger samples sizes within treatments and over longer periods of time are
needed to fully assess the value of native grass seeding mixtures of CRP in the SHP.
90
CP1:
Weeping Lovegrass (WLG)
Old World Bluestem (OWB)
CP2:
Native Shortgrass Prairie:
Native w/ Buffalo Grass (NA-B)
Native w/o Buffalo Grass (NA-A)
Figure 3.1. Location of counties and study fields within the Texas Southern High Plains used in study, 2001 – 2003.
91
Table 3.1. Nesting species composition and the number of avian nests located (proportions in parenthesis) in CRP and native shortgrass prairie cover
types in the Southern High Plains of Texas (2001-2002).
Total
WLG
OWB
Cover TypeA
NAB
Grasshopper Sparrow
35 (29.7)
5 (16.1)
8 (34.8)
16 (64.0)
4 (17.4)
2 (12.5)
13 (23.6)
20 (42.6)
Cassin’s Sparrow
28 (23.7)
6 (19.4)
3 (13.0)
0
8 (34.8)
11 (68.8)
9 (16.4)
8 (17.0)
Mourning Dove
45 (38.1)
17 (54.8)
11 (47.8)
5 (20.0)
10 (43.5)
2 (12.5)
28 (50.9)
15 (31.9)
0
0
3
(12)
1 (4.3)
1 (6.3)
0
4 (8.5)
Western Meadowlark
5 (4.2)
CP TypeA
NAA
NSP
CP1
CP2
Lark Bunting
1 (0.8)
1 (3.2)
0
0
0
0
1 (1.8)
0
Ring-necked Pheasant
2 (1.7)
2 (6.5)
0
0
0
0
2 (3.6)
0
Mallard
2 (1.7)
0
1 (4.3)
1 (4.0)
0
0
1 (1.8)
1 (2.1)
7
5
4
4
4
4
6
5
118
31
23
25
23
16
55
47
Number of Nesting
Species
Total Number of Nests
A
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss;
NAA = native seed mix without buffalograss; NSP = native shortgrass prairie not grazed since 1998 (1
study field in 2001 and 2 study fields in 2002); CP1= introduced grasses (WLG and OWB); CP2 =
introduced grasses (NAB and NAA).
97
Table 3.2. Nesting species composition and number of avian nests located in CRP and native shortgrass prairie cover types in the
Southern High Plains of Texas for 2001 and 2002.
Cover TypeA
NAB
01 02
NAA
01 02
NSP
01 02
CP TypeA
CP1
CP2
01 02
01 02
10
2
2
1
1
4
9
8
12
0
0
2
6
3
8
2
7
2
6
7
1
4
5
5
1
1
12 16
6
9
0
0
1
2
0
1
0
1
0
0
1
3
0
0
0
0
0
0
0
0
0
1
0
0
0
2
0
0
0
0
0
0
0
0
0
2
0
0
0
1
0
0
0
1
1
0
0
0
0
0
0
1
1
0
7
5
5
3
2
4
4
3
3
4
3
4
5
4
5
4
44
74
16
15
5
18
9
16
9
14
5
11
21
33
18
30
Total
01 02
WLG
01 02
OWB
01 02
Grasshopper Sparrow
13
22
3
2
1
7
6
Cassin’s Sparrow
7
21
2
4
0
3
Mourning Dove
19 26
8
9
4
Western Meadowlark
1
4
0
0
Lark Bunting
1
0
1
Ring-necked Pheasant
2
0
Mallard
1
Number of Nesting Species
Total Number of Nests
A
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without
buffalograss; NSP = native shortgrass prairie not grazed since 1998 (1 study field in 2001 and 2 study fields in 2002); CP1=
introduced grasses (WLG and OWB); CP2 = introduced grasses (NAB and NAA).
98
0.9
0.8
0.7
% Total Nests
0.6
0.5
0.4
0.3
0.2
0.1
0
Grasshopper Sparrow
Cassin's Sparrow
Mourning Dove
Total*
Most Common Ground-Nesting Avian Species
>15 May - 09 Jun
10 Jun - 05 Jul
08 Jul - 31 Jul
* Total includes nests from all species
Figure 3.2. Nest initiation dates for ground-nesting birds in the principle grassland cover types in the
Southern High Plains of Texas, 2001 – 2002.
99
100
Table 3.3. Nest densities (n / ha) for nesting species (> 5 nests) in CRP and native shortgrass prairie cover types for 2001, 2002, and years combined in
the Southern High Plains of Texas.
Cover TypeA, B
CP TypeA
WLG
OWB
NAB
NAA
NSP
CP1
CP2
Grasshopper Sparrow
2001
0.25 + 0.14
0.08 + 0.08
0.38 + 0.07
0.13 + 0.07
0
0.17 + 0.08 0.25 + 0.07
2002
Years Combined
0.08 + 0.08
0.17 + 0.08
0.33 + 0.08
ab
0.21 + 0.08
0.44 + 0.21
ab
0.41 + 0.14
0.06 + 0.06
a
0.09 + 0.06
b
0
0.21 + 0.08
0.25 + 0.13
0
0.19 + 0.05
0.25 + 0.09
Cassin’s Sparrow
2001
0.08 + 0.08
0
0
0.13 + 0.13
0.75
0.04 + 0.04
0.06 + 0.06
2002
0.17 + 0.17
0.25 + 0.25
0
0.13 + 0.07
0.75 + 0.25
0.21 + 0.14
0.06 + 0.04
Years Combined
0.13 + 0.13
0.13 + 0.13
0
0.13 + 0.09
0.58 + 0.22
0.13 + 0.08
0.06 + 0.05
2001
0.42 + 0.17
0.25 + 0.14
0.06 + 0.06
0.19 + 0.12
0
0.33 + 0.11
0.13 + 0.07
2002
0.42 + 0.08
0.25 + 0.14
0.19 + 0.12
0.25 + 0.14
0
0.33 + 0.08
0.22 + 0.09
Years Combined
0.42 + 0.11
0.25 + 0.07
0.13 + 0.07
0.22 + 0.11
0
0.33 + 0.07
0.17 + 0.06
2001
0.83 + 0.36
0.33 + 0.08
0.44 + 0.12
0.44 + 0.12
0.75
0.58 + 0.20
0.44 + 0.08
2002
0.67 + 0.30
0.83 + 0.36
0.69 + 0.26
0.44 + 0.12
0.75 + 0.25
0.75 + 0.21
0.56 + 0.14
Years Combined
0.75 + 0.31
0.58 + 0.15
0.56 + 0.19
0.44 + 0.11
0.58 + 0.22
0.67 + 0.16
0.50 + 0.10
Mourning Dove
Total Abundance
A
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with buffalograss; NAA = native seed mix without buffalograss;
NSP = native shortgrass prairie not grazed since 1998 (1 study field in 2001 and 2 study fields in 2002); CP1= introduced grasses (WLG and OWB);
CP2 = introduced grasses (NAB and NAA).
B
Cover type means with the same lowercase letter are not different (P > 0.05).
101
Table 3.4. General nesting biology (x̄ + SE) of Grasshopper Sparrow, Cassin’s
Sparrow, and
Mourning Dove in CRP fields and native shortgrass prairie in the Southern High Plains
of
Texas, 2001-2002.
Grasshopper
Sparrow
(N = 35)
Cassin’s
Sparrow
(N = 28)
Mourning
Dove
(N = 45)
Successful Nest (N)
19
16
21
Depredated Nest (N)
16
12
24
Abandoned Nests (N)A
a) during nest building
b) with eggs
c) parasitized
0
2
0
2
0
1
0
1
0
Mayfield Nest Success
44.7
50.9
31.5
Apparent Nest Success
54.3
57.1
46.7
4.38 + 0.11 (28)
4.14 + 0.10
(24)
1.98 + 0.02
(40)
Hatching Success (N) B
92.9 (19)
90.3 (11)
93.7 (25)
Young Fledged/Nest
1.7 + 0.02
1.6 + 0.02
0.6 + 0.01
3.74 + 0.17
3.63 + 0.13
1.65 + 0.11
3.34
3.69
1.04
General Nesting Data:
Nesting Success:
Nesting Biology:
Clutch Size
Young Fledged/Successful
Nest
Nest Productivity C
A
Not included in the total number of nests
Percent hatched eggs from all eggs for which the clutch size was know with certainty
C
Nest productivity = Mayfield nest success x young fledged per successful nest x
number of broods
per female per year
B
96
Table 3.5. Mayfield nest success of the most common ground nesting birds (> 5 nests) in CRP fields
and native shortgrass prairie in the Southern High Plains of Texas, 2001-2002.
Nest Cycle
Interval
N
Depredated
(n)
Exposure
Days
Survival + SE
Success (%)
Incubation
Nestling
Total
9
10
12
2
3
5
60
68
128
0.98 + 0.02
0.96 + 0.02
0.97 + 0.01
50.9 < 75.7 < 100
41.2 < 66.7 < 100
26.9 < 50.3 < 92.3
2002
Incubation
Nestling
Total
17
17
21
4
6
10
121.5
114.5
236
0.97 + 0.02
0.95 + 0.02
0.96 + 0.01
47.6 < 69.2 < 99.5
41.1< 61.6 < 90.7
24.1 < 42.1 < 72.1
Years Combined
Incubation
Nestling
Total
26
27
35
7
9
16
213.5
191.5
405
0.97 + 0.01
0.95 + 0.02
0.96 + 0.01
52.3 < 69.3 < 91.1
48.3 < 64.8 < 86.2
29.7 < 44.7 < 66.7
Incubation
Nestling
Total
6
6
7
1
2
3
53
46
99
0.98 + 0.02
0.95 + 0.03
0.97 + 0.02
52.9 < 81.1 < 100
37.4 < 67.0 < 100
26.2 < 54.0 < 100
2002
Incubation
Nestling
Total
17
18
21
3
6
9
130.5
133.5
264
0.98 + 0.01
0.96 + 0.02
0.97 + 0.01
57.4 < 77.4 < 100
46.9 < 61.1 < 92.1
31.3 < 50.0 < 78.9
Years Combined
Incubation
Nestling
Total
23
24
28
4
8
12
183.5
179.5
363
0.98 + 0.01
0.95 + 0.02
0.97 + 0.01
61.4 < 78.5 < 99.7
49.0 < 66.0 < 88.1
34.3 < 50.9 < 74.9
2001
Incubation
Nestling
Total
18
12
19
7
5
12
110
112
222
0.94 + 0.02
0.96 + 0.02
0.96 + 0.02
19.5 < 39.8 < 78.6
29.1 < 52.8 < 92.4
8.47 < 21.1 < 51.1
2002
Incubation
Nestling
Total
23
19
26
6
6
12
154
217.5
371.5
0.96 + 0.02
0.97 + 0.01
0.97+ 0.01
36.1 < 57.3 < 89.7
48.9 < 67.6 < 92.7
23.3 < 39.9 < 67.5
Years Combined
Incubation
Nestling
Total
41
32
45
13
11
24
154
217.5
371.5
0.95 + 0.01
0.97 + 0.01
0.96 + 0.01
33.1 < 49.3 < 72.6
46.5 < 62.2 < 82.6
19.6 < 31.5 < 50.3
Grasshopper Sparrow
2001
Cassin’s Sparrow
2001
Mourning Dove
97
Table 3.6. Overall nest success for the most common nesting species (> 5 nests) in 5 principle cover
typesA and CRP CP (conservation practice) typesA (2001-2002 combined) in the Southern High
Plains of Texas.
Species
Cover Type
N
Failed
Mayfield Nest Success (%)
Apparent Success (%)
GRSP
WLG
6
3
9.7 < 34.6 < 100
50.0
OWB
8
5
9.5 < 29.3 < 85.33
37.6
NAB
15
5
38.6 < 60.7 < 94.4
66.7
NAA
4
2
10.1 < 39.4 < 100
50.0
CP1
14
8
13.6 < 31.3 < 70.3
42.9
CP2
19
7
35.6 < 55.7 < 86.3
63.2
NSP
2
2
100
100
WLG
6
3
14.7 < 41.6 < 100
50.0
OWB
3
0
100
100
NAB
0
0
NA
NA
NAA
8
4
15.7 < 40.1 < 98.0
50.0
CP1
9
3
34.4 < 61.3 < 100
66.7
CP2
8
4
15.7 < 40.1 < 98.0
50.0
NSP
11
5
27.3 < 50.6 < 92.2
54.6
WLG
17
8
21.9 < 41.3 < 76.6
52.9
OWB
11
6
8.4 < 25.9 < 76.4
45.5
NAB
5
3
3.2 < 20.8 < 100
40.0
NAA
10
6
5.4 < 20.5 < 76.2
40.0
CP1
28
14
20.2 < 35.4 < 61.3
50.0
CP2
15
9
8.9 < 23.6 < 60.8
40.0
NSP
2
1
5.4 < 20.5 < 76.2
50.0
CASP
MODO
A
WLG = weeping lovegrass; OWB = Old World bluestem; NAB = native seed mix with
buffalograss; NAA = native seed mix without buffalograss; NSP = native shortgrass prairie not
grazed since 1998 (1 study field in 2001 and 2 study fields in 2002); CP1= introduced grasses
(WLG and OWB); CP2 = introduced grasses (NAB and NAA).
98
Table 3.7. Nest site characteristics among the most common ground nesting bird species (> 5 nests) in CRP fields and native shortgrass prairie in
the Southern High Plains of Texas, 2001-2002.
Variable
Grass Cover (%)
Grasshopper
Sparrow
(n = 35)
78.3 + 3.02 a
Cassin’s
Sparrow
(n = 28)
62.3 + 5.29 b
Mourning
Dove
(n = 45)
72.2 + 2.29 ab
F
4.9
df
2, 105
P
0.0089
Forb Cover (%)
4.4 + 1.99 a
18.2 + 5.08 b
5.1 + 1.30 a
7.1
2, 103
0.0013
Bare Ground Cover (%)
11.4 + 1.85
13.9 + 2.90
8.6 + 1.11
2.0
2, 103
0.1355
Litter Cover (%)
5.9 + 1.58 a
3.7 + 0.84 a
14.0 + 1.82 b
11.5
2, 103
0.00003
Litter Depth (cm)
0.4 + 0.06 a
0.5 + 0.07 a
0.8 + 0.07 b
10.9
2, 103
0.00005
Vertical Density (dm)
1.2 + 0.10
1.5 + 0.11
1.6 + 0.14
1.7
2, 105
0.0.643
Average Veg Height (dm)
3.0 + 0.15
3.5 + 0.30
3.8 + 0.21
3.1
2, 105
0.0509
Max. Forb Height (cm)
29.4 + 3.12
32.4 + 3.90
23.1 + 2.30
2.6
2, 79
0.0780
Max. Living Height (cm)
49.1 + 4.05 a
52.8 + 4.64 a
73.1 + 3.62 b
11.1
2, 105
0.00008
Max. Dead Height (cm)
40.0 + 3.41 a
54.9 + 5.20 b
56.9 + 3.44 b
5.3
2, 105
0.0069
Nest Ht. Above Ground (cm)
0.2 + 0.11 a
4.0 + 0.89 b
0.0 + 0.00 a
28.1
2, 105
< 0.00001
Nearest Perch Site (m)
26.1 + 3.94 a
11.9 + 2.49 b
23.4 + 4.035 ab
6.1
2, 46
0.0044
Shrub Density w/i 50 m
4.0 + 1.82 a
20.4 + 3.41 b
4.0 + 1.09 a
19.3
2, 105
< 0.00001
* Means with the same letter are not significantly different (P > 0.05, Tukey’s HSD Test)
99
Table 3.8. Grasshopper Sparrow nest site characteristics between CP1 and CP2 fields in the Southern High Plains
of Texas, 2001-2002
CP1
(n = 13)
CP2
(n = 20)
t
df
P
Grass Cover (%)
76.5 + 5.08
83.8 + 2.51
1.4
31
0.1693
Forb Cover (%)
1.9 + 1.17
1.8 + 0.81
-0.1
31
0.9011
Bare Ground Cover (%)
15.4 + 3.72
8.5 + 1.69
-1.9
31
0.0681
Litter Cover (%)
6.2 + 2.11
6.0 + 2.44
-0.
31
0.9650
Litter Depth (cm)
0.4 + 0.07
0.4 + 0.09
0.5
31
0.6448
Vertical Density (dm)
1.5 + 0.19
1.2 + 0.13
-1.3
31
0.1899
Average Veg Height (dm)
3.6 + 0.26
3.0 + 0.23
-1.7
31
0.0932
Max. Forb Height (cm)
24.1 + 5.12
31.9 + 4.38
1.1
21
0.2871
Max. Living Height (cm)
66.0 + 5.51
42.6 + 3.20 **
-3.9
31
0.0004
Max. Dead Height (cm)
42.5 + 4.07
41.1 + 5.09
-0.2
31
0.8361
Nearest Perch Site (m)
22.9 + 5.15
37.6 + 4.80
1.9
11
0.0781
Shrub Density w/i 50 m
6.5 + 3.86
0.6 + 0.36
-1.9
31
0.0673
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
100
Table 3.9. Grasshopper Sparrow nest site characteristics between successful and depredated nests in CRP fields in the
Southern High Plains of Texas, 2001-2002
Successful
(n = 19)
Depredated
(n = 14)
t
df
P
Grass Cover (%)
77.9 + 4.18
85.0 + 1.57
1.4
31
0.1708
Forb Cover (%)
2.1 + 0.86
1.4 + 1.07
-0.5
31
0.6223
Bare Ground Cover (%)
12.9 + 2.80
8.9 + 2.09
-1.1
31
0.2966
Litter Cover (%)
7.1 + 2.70
4.6 + 1.50
-0.7
31
0.4753
Litter Depth (cm)
0.5 + 0.09
0.3 + 0.06
-1.2
31
0.2396
Vertical Density (dm)
1.1 + 0.13
1.6 + 0.16 *
2.4
31
0.0251
Average Veg Height (dm)
2.9 + 0.17
3.7 + 0.34 *
2.3
31
0.0299
Max. Forb Height (cm)
31.0 + 4.57
25.8 + 4.80
-0.7
21
0.4737
Max. Living Height (cm)
49.1 + 4.69
55.5 + 5.29
0.9
31
0.3747
Max. Dead Height (cm)
38.6 + 3.73
45.7 + 6.33
1.0
31
0.3156
Nearest Perch Site (m)
29.4 + 7.90
29.4 + 4.16
-0.2
11
0.8257
Shrub Density w/i 50 m
1.2 + 0.59
5.2 + 3.63
1.3
31
0.2107
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
101
Table 3.10. Grasshopper Sparrow nest site characteristics between successful and depredated nests in CP1 and CP2 fields in the Southern High Plains
of Texas, 2001-2002
Successful
(n = 6)
CP1
Depredated
(n = 7)
Grass Cover (%)
69.2 + 10.28
Forb Cover (%)
CP2
Depredated
(n = 7)
t
df
P
t
df
P
SuccessfulCP2
(n = 13)
82.9 + 2.40
1.4
11
0.1900
81.9 + 3.69
87.1 + 1.84
1.0
18
0.3346
3.3 + 2.47
0.7 + 0.46
-1.1
11
0.2841
1.5 + 0.60
2.1 + 2.14
0.3
18
0.7332
Bare Ground Cover (%)
19.6 + 7.26
11.8 + 3.02
-1.0
11
0.3167
9.8 + 2.15
6.1 + 2.66
-1.1
18
0.3029
Litter Cover (%)
7.9 + 3.44
4.6 + 2.70
-0.8
11
0.4636
6.7 + 3.69
4.6 + 1.58
-0.4
18
0.6941
Litter Depth (cm)
0.4 + 0.13
0.3 + 0.08
-0.8
11
0.4417
0.5 + 0.12
0.3 + 0.11
-0.8
18
0.4233
Vertical Density (dm)
1.2 + 0.35
1.7 + 0.16
1.2
11
0.2640
1.0 + 0.12
1.5 + 0.28
1.8
18
0.0949
Average Veg Height (dm)
3.2 + 0.34
4.0 + 0.34
1.7
11
0.1130
2.8 + 0.18
3.4 + 0.56
1.2
18
0.2425
Max. Forb Height (cm)
19.0 + 6.27
29.3 + 8.10
1.0
6
0.3556
35.4 + 5.33
22.3 + 5.81
-1.4
13
0.1961
Max. Living Height (cm)
68.7 + 9.47
63.7 + 6.83
-0.4
11
0.6734
40.1 + 3.06
47.3 + 7.21
1.1
18
0.2944
Max. Dead Height (cm)
41.8 + 5.84
43.1 + 6.08
0.2
11
0.8806
37.2 + 4.83
48.3 + 11.59
1.0
18
0.3096
Nearest Perch Site (m)
24.2 + 2.62
20.7 + 14.85
-0.3
6
0.7669
42.5 + 7.50
34.3 + 6.69
-0.8
3
0.4854
Shrub Density w/i 50 m
3.5 + 1.54
9.0 + 7.16
0.7
11
0.5016
0.1 + 0.08
1.4 + 0.97
1.9
18
0.0710
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
102
Table 3.11. Cassin’s Sparrow nest site characteristics between CP1, CP2, and native shortgrass prairie in the Southern High Plains
of Texas, 2001-2002.
Variable
Grass Cover (%)
CP1
(n = 9)
88.3 + 2.89 a
CP2
(n = 8)
61.9 + 6.33 b
Native Prairie
(n = 11)
41.4 + 8.20 b
F
13.3
df
2, 25
P
0.0001
Forb Cover (%)
0.8 + 0.42 a
5.3 + 2.03 a
45.0 + 8.14 b
19.7
2, 25
< 0.0001
Bare Ground Cover (%)
8.3 + 2.36 a
30.3 + 6.06 b
7.1 + 2.21 a
12.3
2, 25
0.0002
Litter Cover (%)
2.5 + 0.72
2.5 + 1.25
6.6 + 1.85
2.8
2, 25
0.0816
Litter Depth (cm)
0.4 + 0.12
0.5 + 0.14
0.5 + 0.10
0.5
2, 25
0.6199
Vertical Density (dm)
2.1 + 0.16 a
1.8 + 0.10 a
1.1 + 0.15 b
12.1
2, 25
0.0002
Average Veg Height (dm)
3.9 + 0.55 ab
4.8 + 0.50 a
2.5 + 0.12 b
8.2
2, 25
0.0019
Max. Forb Height (cm)
15.4 + 3.37 a
27.9 + 5.88 ab
43.1 + 5.36 b
5.9
2, 20
0.0098
Max. Living Height (cm)
63.8 + 6.78 a
68.0 + 7.81 a
38.0 + 2.99 b
8.5
2, 25
0.0016
Max. Dead Height (cm)
64.4 + 7.03 a
61.8 + 8.87 ab
39.3 + 5.96 b
4.1
2, 25
0.0300
5.4 + 2.10
5.1 + 1.49
2.2 + 0.91
1.5
2, 25
0.2470
Nearest Perch Site (m)
13.2 + 2.54 ab
26.2 + 8.72 a
4.3 + 0.685 b
9.1
2, 22
0.0014
Shrub Density w/i 50 m
13.7 + 3.42 a
2.9 + 1.01 a
38.6 + 3.55 b
35.5
2, 25
< 0.0001
Nest Ht. Above Ground (cm)
* Means with the same letter are not significantly different (P > 0.05, Tukey’s HSD Test)
103
Table 3.12. Cassin’s Sparrow nest site characteristics between successful and depredated nests in CRP fields and
native shortgrass prairie in the Southern High Plains of Texas, 2001-2002
Successful
(n = 16)
Depredated
(n = 12)
t
df
P
Grass Cover (%)
59.1 + 7.83
66.7 + 6.78
0.7
26
0.4875
Forb Cover (%)
23.0 + 7.15
14.8 + 7.13
-0.8
26
0.4353
Bare Ground Cover (%)
13.0 + 3.95
15.6 + 4.04
0.5
26
0.6479
Litter Cover (%)
5.0 + 1.41
2.9 + 0.91
-1.1
26
0.2607
Litter Depth (cm)
0.5 + 0.07
0.5 + 0136
0.2
26
0.8785
Vertical Density (dm)
1.8 + 0.15
1.4 + 0.17
-1.7
26
0.1017
Average Veg Height (dm)
3.4 + 0.28
3.9 + 0.56
0.9
26
0.3993
Max. Forb Height (cm)
31.2 + 6.63
33.8 + 4.10
0.3
21
0.7425
Max. Living Height (cm)
56.2 + 5.52
53.1 + 6.48
-0.4
26
0.7177
Max. Dead Height (cm)
51.6 + 6.59
56.8 + 6.21
0.6
26
0.5832
Nest Height Above Ground (cm)
4.9 + 1.32
3.0 + 1.08
-1.1
26
0.3003
Nearest Perch Site (m)
14.6 + 3.94
7.9 + 1.48
-1.3
23
0.1970
Shrub Density w/i 50 m
19.8 + 4.72
21.1 + 5.09
0.2
26
0.8575
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
104
Table 3.13. Cassin’s Sparrow nest site characteristics between successful and depredated nests in CP1 and CP2 fields and native shortgrass
prairie in the Southern High Plains of Texas, 2001-2002
Successful
(n = 6)
CP1
Depredated
(n = 3)
t
df
P
Successful
(n =4)
Grass Cover (%)
89.2 + 4.17
86.7 + 3.33
0.7
7
0.7110
60.0 + 12.42
Forb Cover (%)
0.8 + 0.53
0.8 + 0.83
-0.0
7
1.0
Bare Ground Cover (%)
7.5 + 3.35
10.0 + 2.89
0.5
7
Litter Cover (%)
2.5 + 0.91
2.5 + 1.44
0.0
Litter Depth (cm)
0.4 + 0.16
0.3 + 0.22
Vertical Density (dm)
2.2 + 0.17
Average Veg Height (dm)
Variable
CP2
Depredated
(n = 4)
t
df
P
63.8 + 5.54
0.3
6
0.7920
5.6 + 0.3.29
5.0 + 2.89
-0.1
6
0.8911
0.6491
30.6 + 10.82
30.0 + 7.36
-0.0
6
0.9635
7
1.0
3.8 + 2.17
1.3 + 1.25
-1.0
6
0.3559
-0.2
7
0.8517
0.5 + 0.05
0.5 + 0.31
0.1
6
0.9074
1.7 + 0.27
-1.7
7
0.1308
1.8 + 0.11
1.8 + 0.18
0.1
6
0.8864
3.5 + 0.44
4.5 + 1.53
0.8
7
0.4470
4.3 + 0.67
5.3 + 0.75
1.0
6
0.3527
Max. Forb Height (cm)
11.5 + 3.50
18.0 + 5.03
0.9
3
0.4217
17.5 + 3.71
41.7 + 6.89 *
3.3
5
0.0206
Max. Living Height (cm)
62.0 + 7.77
67.3 + 15.5
0.4
7
0.7367
74.8 + 10.90
61.3 + 11.63
-0.8
6
0.4296
Max. Dead Height (cm)
58.2 + 9.72
77.0 + 1.53
1.3
7
0.2284
59.5 + 15.06
64.0 + 11.69
0.2
6
0.8212
Nest Ht. Above Ground (cm)
6.8 + 2.84
2.7 + 2.67
-0.9
7
0.3916
6.0 + 2.02
4.1 + 0.2.38
-0.6
6
0.5755
Nearest Perch Site (m)
13.5 + 3.75
12.7 + 2.67
-0.1
7
0.8890
39.0 + 6.65
7.1 + 3.95 *
-3.5
3
0.0390
Shrub Density w/i 50 m
12.8 + 4.56
15.3 + 5.84
0.3
7
0.7546
2.8 + 1.03
3.0 + 1.91
0.1
6
0.9122
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
105
Table 3.13. Continued. Cassin’s sparrow nest site characteristics between successful and depredated nests
Successful
(n = 6)
Native Prairie
Depredated
(n = 5)
t
df
P
Grass Cover (%)
28.3 + 6.41
57.0 + 14.02
2.0
9
0.0795
Forb Cover (%)
56.7 + 6.28
31.0 + 14.70
-1.7
9
0.1207
Bare Ground Cover (%)
6.7 + 3.33
7.5 + 3.16
0.2
9
0.8621
Litter Cover (%)
8.3 + 3.07
4.5 + 1.66
-1.0
9
0.3288
Litter Depth (cm)
0.5 + 0.10
0.6 + 0.19
0.1
9
0.8872
Vertical Density (dm)
1.3 + 0..22
0.9 + 0.19
-1.5
9
0.1794
Average Veg Height (dm)
2.6 + 0..20
2.4 + 0.13
-1.0
9
0.3571
Max. Forb Height (cm)
46.8 + 9.34
38.6 + 4.24
-0.7
9
0.4737
Max. Living Height (cm)
38.0 + 4.14
38.0 + 4.83
0.0
9
1.0
Max. Dead Height (cm)
39.7 + 10.83
38.8 + 4.43
-0.1
9
0.9468
Nest Height Above Ground (cm)
2.2 + 1.38
2.2 + 1.29
0.0
9
0.9865
Nearest Perch Site (m)
3.4 + 0.79
5.4 + 1.03
1.6
9
0.1482
Shrub Density w/i 50 m
38.2 + 6.39
39.0 + 2.83
0.1
9
0.9141
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
106
Table 3.14. Mourning Dove nest site characteristics between CP1 and CP2 fields in the Southern High Plains
of Texas, 2001-2002
CP1
(n = 28)
CP2
(n = 15)
t
df
P
Grass Cover (%)
79.11 + 2.23
61.67 + 3.80 **
4.24
41
0.0001
Forb Cover (%)
2.50 + 0.59
8.50 + 3.16 *
-2.47
41
0.0177
Bare Ground Cover (%)
7.23 + 1.26
10.67 + 2.16
-1.47
41
0.1480
Litter Cover (%)
11.16 + 1.63
19.17 + 4.21 *
-2.11
41
0.0406
Litter Depth (cm)
0.78 + 0.09
0.82 + 0.14
-0.27
41
0.7852
Vertical Density (dm)
1.96 + 0.15
1.14 + 0.22 **
3.82
41
0.0004
Average Veg Height (dm)
4.36 + 0.19
3.19 + 0.22 **
3.89
41
0.0004
Max. Forb Height (cm)
19.47 + 3.04
27.84 + 3.67
-1.76
30
0.0890
Max. Living Height (cm)
82.57 + 2.03
60.60 + 5.95 **
4.28
41
0.0001
Max. Dead Height (cm)
61.43 + 3.33
45.20 + 4.08 *
2.98
41
0.0048
Nearest Perch Site (m)
23.40 + 3.47
15.00 + 0.00
-0.61
14
0.5542
Shrub Density w/i 50 m
4.96 + 1.49
0.33 + 0.33 *
2.24
41
0.0302
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
107
Table 3.15. Mourning Dove nest site characteristics between successful and depredated nests in CRP fields in
the Southern High Plains of Texas, 2001-2002
Successful
(n = 19)
Depredated
(n = 24)
t
df
P
Grass Cover (%)
72.1 + 3.77
73.8 + 2.97
0.3
41
0.7298
Forb Cover (%)
4.7 + 2.39
4.5 + 1.17
-0.1
41
0.9183
Bare Ground Cover (%)
7.1 + 1.67
9.5 + 1.51
1.0
41
0.3005
Litter Cover (%)
16.1 + 2.98
12.3 + 2.40
-1.0
41
0.3257
Litter Depth (cm)
0.9 + 0.12
0.7 + 0.07
1.5
41
0.1472
Vertical Density (dm)
1.7 + 0.17
1.6 + 0.16
-0.6
41
0.5816
Average Veg Height (dm)
4.2 + 0.28
3.7 + 0..19
-1.6
41
0.1129
Max. Forb Height (cm)
21.5 + 3.49
23.8 + 3.36
0.5
30
0.6548
Max. Living Height (cm)
75.5 + 4.00
74.4 + 4.21
-0.2
41
0.8523
Max. Dead Height (cm)
56.2 + 4.14
55.4 + 3.95
-0.1
41
0.8911
Nearest Perch Site (m)
17.3 + 4.45
26.2 + 4.35
1.3
14
0.2011
Shrub Density w/i 50 m
2.8 + 1.32
3.8 + 1.54
0.4
41
0.6670
Variable
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
108
Table 3.16. Mourning Dove nest site characteristics between successful and depredated nests in CP1 and CP2 fields in the Southern
High Plains of Texas, 2001-2002
Variable
Successful
(n = 6)
CP1
Depredated
(n = 3)
t
P
Successful
(n =4)
CP2
Depredated
(n = 4)
t
df
df
P
Grass Cover (%)
79.2 + 3.75
79.0 + 2.73
-0.1
26
0.9600
56.7 + 4.41
65.0 + 5.53
1.1
13
0.2995
Forb Cover (%)
1.7 + 0.87
3.2 + 0.79
1.2
26
0.2310
11.3 + 7.00
6.7 + 2.76
-0.7
13
0.4979
Bare Ground Cover (%)
6.3 + 1.76
8.0 + 1.81
0.6
26
0.5215
8.8 + 3.86
11.9 + 2.63
0.7
13
0.4895
Litter Cover (%)
12.7 + 3.23
9.8 + 1.26
-0.9
26
0.3930
23.3 + 5.58
16.4 + 6.03
-0.8
13
0.4394
Litter Depth (cm)
0.9 + 0.14
0.6 + 0.08
1.7
26
0.1010
0.8 + 0.12
0.9 + 0.23
0.3
13
0.7630
Vertical Density (dm)
1.9 + 0.23
2.0 + 0.19
0.4
26
0.6922
1.4 + 0.11
1.0 + 0.13
-2.6
13
0.0219
Average Veg Height (dm)
4.6 + 0.32
4.1 + 0.19
-1.5
26
0.1493
3.4 + 0.37
3.1 + 0.28
-0.7
13
0.5082
Max. Forb Height (cm)
18.3 + 4.25
20.5 + 4.51
0.3
17
0.7325
28.8 + 5.02
27.4 + 4.99
-0.2
11
0.8778
Max. Living Height (cm)
83.2 + 3.58
82.1 + 2.30
-0.3
26
0.7950
59.0 + 6.02
61.7 + 9.38
0.2
13
0.8356
Max. Dead Height (cm)
62.8 + 4.88
60.2 + 4.69
-0.4
26
0.6998
41.8 + 3.30
47.4 + 6.52
0.7
13
0.5208
Nearest Perch Site (m)
17.3 + 4.45
27.4 + 4.66
1.5
13
0.1599
0.0 + 0.00
15.0 + 0.00
Shrub Density w/i 50 m
4.2 + 1.83
5.7 + 2.33
0.5
26
0.6221
0.0 + 0.00
0.6 + 0.56
0.8
13
0.4346
** Means that remained significantly different after Bonferroni adjustment
* P < 0.05
109
LITERATURE CITED
Anderson, J. T., and W. C. Conway. 2000. The flight song display of the Cassin’s
sparrow (Aimophila cassinii): form and possible function. Bulletin of the Texas
Ornithological Society 33(1):1-5.
Askins, R. A. 1993. Population trends in grassland, shrubland, and forest birds in eastern
North America. Current Ornithology 11:1-34.
Bakker, K. K., D. E. Naugle, and K. F. Higgins. 2002. Incorporating landscape attributes
into models for migratory grassland bird conservation. Conservation Biology
16(6):1638-1646.
Baxter, W. L., and C. W. Wolfe. 1973. Life history and ecology of the ring-necked
pheasant in Nebraska. Nebraska Game and Parks Commision, Lincoln, 58 pp.
Beason, R. C. 1995. Horned Lark (Eremophila alpestris). The birds of North America,
number 195. American Ornithologists’ Union. Washington, D.C., USA., and The
Academy of Natural Sciences, Philadelphia, Pennsylvania, USA.
Berthelesen, P. S. 1989. Value of the Conservation Reserve Program to birds in the Texas
Southern High Plains. M.S. Thesis. Texas Tech Univ., Lubbock. 106 pp.
Berthelesen, P. S., L. M. Smith, and C. L. Coffman. 1989. CRP land and game bird
production in High Plains of Texas. Journal of Soil and Water Conservation.
44:504-507.
Berthelesen, P. S., L. M. Smith, and R. R. George. 1990. Ring-necked pheasant nesting
ecology and production on CRP lands in the Texas Southern High Plains.
Transactions of the North American Wildlife and Natural Resources Conference.
55:46-56.
Berthelesen, P. S. and L. M. Smith. 1995. Nongame bird nesting on CRP lands in the
Texas southern High Plains. Journal of Soil and Water Conservation. 50:672-675.
Best, L. B., H. Campa, III, K. E. Kemp, R. J. Robel, M. R. Ryan, J. A. Savidge, H. P.
Weeks, Jr., and S. R. Winterstein. 1997. Bird abundance and nesting in CRP
fields and cropland in the Midwest: a regional approach. Wildlife Society Bulletin
25:864-877.
Best, L. B., H. Campa, III, K. E. Kemp, R. J. Robel, M. R. Ryan, J. A. Savidge, H. P.
Weeks, Jr., and S. R. Winterstein. 1998. Avain abundance in CRP and crop fields
during winter in the Midwest. American Midland Naturalist. 139:311-324.
110
Blackstock, D. A. 1979. Soil survey: Lubbock County, Texas. USDA Soil
Conservation Service. Washington, D.C.
Blakely, E. R., and W. M. Koos. 1974. Soil survey: Hale County, Texas. USDA Soil
Conservation Service. Washington, D.C.
Bolen, E.G., L.M. Smith, and H.L. Schramm, Jr. 1989. Playa lakes: prairie wetlands of
the Southern High Plains. BioScience 39:615-623.
Bollinger, E. K. 1995. Successional changes and habitat selection in hayfield bird
communities. Auk 112:720-730.
Bollinger, E.K., P.B. Bollinger, and T.A. Gavin. 1990. Effects of hay-cropping on eastern
populations of the bobolink. Wildlife Society Bulletin 18:142-150.
Brady, S. 2000. Wildlife Response to the Conservation Reserve Program. Farm Bill
Workshop, July 8-10, 2000. http://www.fb-net.org/FB/worksop/CRP-wild.htm.
Buckland, S. T., D. R. Anderson, K. P. Burnham, J. L. Laake, D. L. Borchers, L. Thomas.
2001. Introduction to distance sampling: estimating abundance of biological
populations. Oxford University Press.
Burnham, K. P., D. R. Anderson, and J. L. Laake. 1980. Estimation of density from line
transect sampling of biological populations. Wildlife Monograph 72.
Carrie, N. R., R. O. Wagner, K. R. Moore, J. C. Sparks, E. L. Keith, and C. A. Melder.
2002. Winter abundance of and habitat use by Henslow’s Sparrows in Louisiana.
Wilson Bulletin 114(2): 221-226.
Christoferson, L. L., and M. L. Morrison. 2001. Integrating methods to determine
breeding and nesting status of 3 western songbirds. Wildlife Society Bulletin
29(2):688-696.
Cody, M. L. 1968. On the methods of resource division in grassland bird communities.
American Naturalist 102:107-147.
Conover, W. J. 1980. Practical nonparametric statistics. John Wiley and Sons, Toronto,
ON.
Coppedge, B. R., D. M. Engle, R. E. Masters, and M. S. Gregory. 2001. Avian response
to landscape change in fragmented southern Great Plains grasslands. Ecological
Applications 11(1):47-59.
111
Daubenmire, R. 1959. A canopy-coverage method of vegetational analysis. Northwest
Science 33(1):43-64.
Davis, S. K. and D. C. Duncan. 1999. Grassland songbird occurrence in native and
crested wheatgrass pastures of southern Saskatchewan. Studies in Avian Biology
19:211-218.
Delisle, J. M. and J. A. Savidge. 1997. Avian use and vegetation characteristics of
Conservation Reserve Program fields. Journal of Wildlife Management
61(2):318-325.
Dunning, Jr., J. B., R. K. Bowers, Jr., S. J. Suter, and C. E. Bock. 1999. Cassin’s Sparrow
(Aimophila cassinii). The birds of North America, number 471. American
Ornithologists’ Union. Washington, D.C., USA., and The Academy of Natural
Sciences, Philadelphia, Pennsylvania, USA.
Ehrlich, P. R., D.S. Dobkin, and D. Wheye. 1988. The birder’s handbook: a field guide to
the natural History of North American birds. Simon & Schuster, New York, New
York, USA.
Fletcher, R. J. Jr., and R. R. Koford. 2002. Habitat and landscape associations of breeding
birds in native and restored grasslands. Journal of Wildlife Management
66(4):1011-1022.
Fuhendorf, S. D., and D. M. Engle. 2001. Restoring heterogeneity on rangelands:
ecosystem management based on evolutionary grazing patterns. BioScience
51(8):625-632.
George, R. R., A. L. Farris, C. C. Schwartz, D. D. Humburg, and J. C. Coffey. 1979.
Native prairie grass pastures as nest cover for upland birds. Wildlife Society
Bulletin 7(1):4-8.
Gransfors, D. A., K. E. Church, and L. M. Smith. 1996. Eastern meadowlark nesting in
Rangelands and Conservation Reserve Program fields in Kansas. Journal of Field
Ornithology 67:222- 235.
Gryzbowski, J. A. 1982. Population structure in grassland bird communities during
winter. Condor 84:137-152.
Haukos, D. A., and L. M. Smith. 1997. Common Flora of the Playa Lakes. Texas Tech
University Press, Lubbock Texas.
112
Heard, L. P. 2000. Introduction. In A Comprehensive Review of Farm Bill Contributions
to Wildlife Conservation. USDA-NRCS Technical Report. Pp: 1-5.
Herkert, J.R. 1998. The influence of the CRP on grasshopper sparrow population trends
in the mid-continental United States. Wildlife Society Bulletin 26(2):227-231.
Holliday, V. T. 1991. The geologic record of wind erosion, eolian deposition, and aridity
on the Southern High Plains. Great Plains Research 1:7-25.
Hughes, J. P., R. J. Robel, and K. E. Kemp. 2000. Factors influencing Mourning Dove
nest success in CRP Fields. Journal of Wildlife Management 64(4):1004-1008.
Hull, S. D., R. J. Robel, and K. E. Kemp. 1996. Summer avian abundance, invertebrate
biomass, and forbs in Kansas CRP. Prairie Naturalist 28(1):1-12.
Hutto, R. L., and S. M. Pletschet, and P. Hendricks. 1986. A fixed-radius point count
method for non-breeding and breeding season use. Auk 103:593-602.
Igl, L. D., and B. M. Ballard. 1999. Habitat associations of migrating and overwintering
grassland birds in Southern Texas. Condor 101:771-782.
Johnson, D. H. 2001. Habitat fragmentation effects on birds in grasslands and wetlands: a
critique of our knowledge. Great Plains Research 11(2):211-213.
Johnson, D. H. 1979. Estimating nest success: the Mayfield method and an alternative.
Auk 96:651-661.
Johnson, D. H. 1980. The comparison of usage and availability measurements for
evaluating resource preference. Ecology 61:65-75.
Johnson, D. H., and L. D. Igl. 2001. Area requirements of grassland birds: a regional
perspective. Auk 118(1):24-34.
Johnson, D. H., and M. D. Schwartz. 1993a. The Conservation Reserve Program:
habitat for grassland birds. Great Plains Research 3:273-295.
Johnson, D. H., and M. D. Schwartz. 1993b. The Conservation Reserve Program and
grassland birds. Conservation Biology 7:934-937.
King, J. W. and J. A. Savidge. 1995. Effects of the Conservation Reserve Program on
wildlife in southeast Nebraska. Wildlife Society Bulletin 23(3):377-385.
113
Klute, D. S., R. J. Robel, and K. E. Kemp. 1997. Seed availability in grazed pastures and
Conservation Reserve Program fields during winter in Kansas. Journal of Field
Ornithology 68(2):253-258.
Knopf, F. L. 1994. Avian assemblages on altered grasslands. Studies in Avian Biology
15:247-257.
Koford, R. R. 1999. Density and fledging success of grassland birds in Conservation
Reserve Program fields in North Dakota and west-central Minnesota. Studies in
Avian Biology 19:187-195.
Kostecke, R., A. Floyd, and C. Stognes. 2001. Checklist of the Birds of the Texas South
Plains. Seventh edition. Llano Estacado Audubon Society.
Krebs, C. J. 1999. Ecological methodology. Second edition. Addison-Welsey Educational
Publishers, Inc. Menlo Park, CA. 620pp.
Levene, H. 1960. Robust tests for equality of variances. Pages 278-292 in T. Oklin, ed.
Contributions to probability and statistics. Stanford University Press, Palo Alto,
California.
Magurran, A. E. 1988. Ecological diversity and its measurement. Princeton University
Press, Princeton, New Jersey.
Martin, T. E., and C. J. Conway. 1994. BBIRD field protocol: breeding biology research
and monitoring database. Montana Cooperative Wildlife Research Unit,
University of Montana, Missoula, USA.
Martin, T. E., and G. R. Geupel. 1993. Nest-monitoring plots: methods for locating nests
and monitoring success. Journal of Field Ornithology 64(4): 507-519.
Maurer, B. A. 1986. Predicting habitat quality for grassland birds using density-habitat
correlations. Journal of Wildlife Management 50(4):556-566.
Mayfield, H. F. 1961. Nesting success calculated from exposure. Wilson Bull. 73:255261.
Mayfield, H. F. 1975. Suggestions for calculating nest success. Wilson Bulletin 87:456466.
McCoy, T. D., M. R. Ryan, E. W. Kurzejeski, and L. W. Burger. 1999. Conservation
Reserve Program: source or sink habitat for grassland birds in Missouri. Journal
of Wildlife Management 63:530-538.
114
McCoy, T. D., M. R. Ryan, L.W. Burger, and E. W. Kurzejeski. 2001. Grassland bird
conservation: CP1 vs. CP2 plantings in conservation reserve program fields in
Missouri. American Midland Naturalist 145:1-17.
McIntyre, N. E., and T. R. Thompson. 200-. A comparison of Conservation Reserve
Program habitat plantings with respect to arthropod prey for declining grassland
birds. American Midland Natrualist (?)
Mikol, S. A. 1980. Field guidelines for using transects to sample nongame bird
populations. U. S. Gov. Printing Off., Washington, D.C. 24pp.
Millenbah, K.F., S.R. Winterstein, H. Campa III, L.T. Furrow, and R.B. Minnis. 1996.
Effects of conservation reserve program field age on avian relative abundance,
diversity, and productivity. Wilson Bulletin 108(4):760-770.
National Oceanic and Atmospheric Administration. 2003. National Climatic Data Center
home page: climate division precipitation, temperature, and drought data.
Www.ncdc.noaa.go/onlineprod/drought/xmgr.html.
Noss, R. F., E. T. Laroe, and J. M. Scott. 1995. Endangered ecosystems of the United
States: a preliminary assessment of loss and degradation. National Biological
Service Report No. 0611-R-01 (MF). U.S. Department of the Interior,
Washington, D.C.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin, Texas.
Patterson, M. P., and L. B. Best. 1996. Bird abundance and productivity in Iowa CRP
fields: the importance of vegetation structure and composition. American Midland
Naturalist 135:153-167.
Peterjohn, B. G. and J. R. Sauer. 1999. Population status of North American grassland
birds from the North American breeding bird survey, 1966-1996. Studies in Avian
Biology 19:27-44.
Prescott, D. R., R. Arbuckle, B. Goddard, and A. Murphy. 1993. Methods for the
monitoring and assessment of avian communities on NAWMP landscapes in
Alberta, and 1993 results. Alberta North American Waterfowl Management Plan
Centre, Edmonton, AB.
Ralph, C. J., G. R. Geupel, P. Pyle, T. E. Martin, and D. F. DeSante. 1993. Handbook of
field methods for monitoring landbirds. USDA Forest Service GTR PSW-GTR144.USDA Forest Service, Albany, CA.
Reynolds, R. T., J. M. Scott, and R. A. Nussbaum. 1980. A variable circular-plot method
for estimating bird numbers. Condor 82:309-313.
115
Robel, R. J., and N. A. Slade. 1965. The availability of sunflower and ragweed seed
during fall and winter. Journal of Wildlife Management 29:202-206.
Robel, R. J., J. N. Briggs, A. D. Dayton, and L. C. Hulbert. 1970. Relationship between
visual obstruction measurements and weight of grassland vegetation. J. Range
Manage. 23:295-297.
Rotella, J.J., E.M. Madden, and A.J. Hansen. 1999. Sampling considerations for
estimating density of passerines in grasslands. Studies in Avian Biology 19:237243.
Rottenberry, J. T., and J. A. Weins. 1980. Habitat structure, patchiness, and avian
communities in North American steppe vegetation: a multivariate analysis.
Ecology 61:1228-1250.
Rottenberry, J. T., and J. A. Weins. 1991. Weather and reproductive variation in
shrubsteppe sparrows: a hierarchical analysis. Ecology 72(4):1325-1353.
Ruth, J. M. 2000. Cassin’s sparrow status assessment and conservation plan. USGS
Midcontinent Ecological Scinece Center. BTP-R6002-2000. Fort Collins, CO.
Samson, F. and F. Knopf. 1994. Prairie conservation in North America. BioScience
44(6):418-421.
Sauer, J.R., S. Schwartz, B.G. Peterjohn, and J.E. Hines. 1996. The North American
Breeding Bird Survey home page. Version 94.3 Patuxent Wildlife Research
Center, Laurel, Maryland. URL = http://www.mbr.nbs.gov/bbs/bbs.html
Sayre, M. W., and N. J. Silvy. 1993. Nesting and production. Pp. 81-104 in Ecology and
management of the Mourning Dove (T. S. Baskett, M. W. Sayre, R. E. Tomlinson
and R. E. Mirarchi, eds.). Stackpole Books, Harrisburg, PA.
Schneider, N. A. 1998. Passerine use of grasslands managed with two grazing regimes on
the Missouri Coteau in North Dakota. M.S. thesis. South Dakota State University,
South Dakota. 94pp.
Seyffert, K. D. 2001. Birds of the Texas Panhandle: their status, distribution, and history.
Texas A&M University Press, College Station, Texas.
Shapiro, S. S. and M. B. Wilk. 1965. An analysis of variance test for normality (complete
samples). Biometrika 52:591-611.
Smith, J. G., J. L. Outlaw, and E. G. Smith. 1996. Background on the CRP program as of
January 1996. Texas A&M Research and Extension Center. Amarillo, Texas.
116
Sokal, R. R., and F. J. Rohlf. 1995. Biometry. Third edition. W. H. Freeman and
Company, New York, USA.
USDA. 2003. http://www.fsa.usda.go/dafp/cepd/crp.htm.
Van Horne, B. 1983. Density as a misleading indicator of habitat quality. Journal of
Wildlife Management 47:893-901.
Vickery, P. D. 1996. Grasshopper Sparrow (Ammodramus savannarum). The birds of
North America, number 239. American Ornithologists’ Union. Washington, D.C.,
USA., and The Academy of Natural Sciences, Philadelphia, Pennsylvania, USA.
Vickery, P. D., M. L. Hunter, Jr., and J. V. Wells. 1992. Use of a new reproductive index
to evaluate relationship between habitat quality and breeding success. Auk
109:697-705.
Vickery, P. D., M. L. Hunter, Jr., and J. V. Wells. 1992. Is density an indicator of
breeding success? Auk 109:706-710.
Warner, R. E. 1994. Agricultural land use and grassland habitat in Illinois: future shock
for midwestern birds? Conservation Biology 8:147-156.
Wiens, J. A. 1969. An approach to the study of ecological relationships among grassland
birds. Ornithological Monograph 8:1-93.
Wiens, J. A. 1973. Pattern and process in grassland bird communities. Ecololocigal .
Monograph 43:237-270.
Wiens, J. A., and J. T. Rottenberry. 1979. Diet niche relationships among North
American grassland and shurbsteppe birds. Oecologia 42:253-292.
Wildlife Management Institute. 1994. The Conservation Reserve Program: a wildlife
conservation legacy. Wildlife Management Institute, Washington, D.C., USA.
Winter, M. 1999. Nesting biology of Dickcissels and Henslow’s Sparrows in
southwestern Missouri prairie fragments. Wilson Bulletin 111(4):515-527.
Zar, J. H. 1984. Biostatistical analysis. Second ed. Prentice-Hall, Inc., Englewood Cliffs,
N.J. 718pp.
117
APPENDIX
SCIENTIFIC NAMES OF AVIAN AND PLANT SPECIES CITED
118
Table A1. Avian species codes, common names, and scientific names
Code
Common Name
Scientific Name
GRSP
Grasshopper Sparrow
Ammodramus savannarum
CASP
Cassin’s Sparrow
Aimophila cassinii
WEME
Western Meadowlark
Stunella neglecta
MODO
Mourning Dove
Zenaida macroura
WEKI
Western Kingbird
Tyrannus verticalis
LARB
Lark Bunting
Calamospiza melanocorys
RNPH
Ring-necked Pheasant
Phasianus colchicus
BUOR
Bullock’s Oriole
Icterus bullockii
STFC
Scissor-tailed Flycatcher
Tyrannus savana
MALL
Mallard
Anas platyrhynchos
GWTE
Green-winged Teal
Anas crecca
GTGR
Great-tailed Grackle
Quiscalus mexicanus
BUOW
Burrowing Owl
Athene cunicularia
BLGR
Blue Grosbeak
Guiraca caerulea
HOLA
Horned Lark
Eremophila alpestris
SASP
Savannah Sparrow
Passerculus sandwichensis
VESP
Vesper Sparrow
Pooecetes gramincus
NOHA
Northern Harrier
Circus cyaneus
PRFA
Prairie Falcon
Falco mexicanus
SEOW
Short-eared Owl
Asio flammeus
GTTO
Green-tailed Towee
Pipilo chlorurus
119
Table A2. Plant species common and scientific names
Common Name
Scientific Name
Grasses:
Arizona cottontop
Digitaria californica
Blue grama
Bouteloua gracilis
Buffalograss
Buchloe dactyloides
Burrograss
Scleropogon brevifolius
Curley mesquite
Hilaria belangeri
Sand dropseed
Sporobolus cryptandrus
Japanese chess
Bromus japonicus
Johnsongrass
Sorghum halepense
Little bluestem
Schizachyrium scoparium
Old World bluestem
Bothriochloa ischaemum
Kleingrass
Panicum antidotale
Rescuegrass
Bromus catharticus
Sedge
Carex spp.
Silver bluestem
Bothriochloa laguroides
Side-oats grama
Bouteloua curtipendula
Green sprangletop
Leptochloa dubia
Switchgrass
Panicum coloratum
Purple threeawn
Aristida purpurea
Tobossa
Hilaria mutica
Tumblegrass
Schedonnardus paniculatus
Bermudagrass
Cynodon dactylon
Western wheatgrass
Agropyron smithii
Weeping lovegrass
Eragrostis curvula
Windmill grass
Chloris verticillata
Forbs:
Annual aster
Aster subulatus
Plains ironweed
Vernonia marginata
Blueweed sunflower
Helianthus ciliaris
Buffalo spur
Solanum rostratum
120
Table A2. Continued.
Common Name
Scientific Name
Cocklebur
Xanthium strumarium
Devil’s claw
Proboscidea louisinica
Goat’s beard
Tragopogon dubius
Purple ground cherry
Quincula lobata
Illinois bundle-flower
Desmanthus illinoensis
Indian blanket
Gallardia pulchella
Scarlet globemallow
Sphaeralcea coccinea
Kochia
Kochia scoparia
Mexican hat
Ratibida columnaris
Milkweed
Asclepias spp.
Mare’s-tail
Conyza canadensis
Tanseymustard
Descuraninia pinnata
Fourwing saltbush
Atriplex canescens
Prickly Lettuce
Lactuca serriola
Spotted evening primrose
Oenothera canescens
Prairie coneflower
Ratibida columnifera
Rubber rabbitbush
Chrysothamnus nauseosus
Ragweed
Ambrosia spp.
Russian thistle
Salsola kali
Goldenweed
Haplopappus ciliatus
Sensitive briar
Schrankia uncinata
Smartweed
Polygonum spp.
Silver-leaf nightshade
Solanum elaeagnifolium
Curly-top gumweed
Grindelia squarrosa
Common sunflower
Helianthus annuus
Tansey aster
Machaeranthera tanacetifolia
Tumbling aster
Sisymbrium altissimum
Sunflower
Helianthus Spp.
121
Table A2. Continued.
Common Name
Scientific Name
Shrubs:
Yucca
Yucca Spp.
Sand Plum
Prunus texana
Mesquite
Prosopis glandulosa
False willow
Baccharis texana
Hackberry
Celtis spp.
Sand Sagebrush
Artemisia filifolia
Broom snakeweed
Gutierrezia sarothrae
Prickly Pear
Opuntia polycantha
122