AVIAN COMMUNITY RESPONSE TO LARGE-SCALE WILDFIRES IN
THE TEXAS PANHANDLE
By
Anthony J. Roberts, B.S.
A THESIS
IN
WILDLIFE SCIENCE
Submitted to the Graduate Faculty of Texas Tech University and the Department of
Natural Resources Management, in Partial Fulfillment of the Requirements for a
Degree of
MASTER OF SCIENCE
Approved by:
___________________________________
Dr. Clint Boal
__________________________________
Dr. Sandra Rideout-Hanzak
__________________________________
Dr. David Wester
_________________________________
Dr. Mark Wallace
_________________________________
Dean of the Graduate School
May, 2009
Texas Tech University, Anthony Roberts, May 2009
ACKNOWLEDGEMENTS
First I would like to thank all the landowners who gave me access to their
property. Without them none of this work would have been possible. Many thanks go to
Dr. Clint Boal for his help and encouragement throughout my time as a graduate student
here at Texas Tech. I would also like to thank Dr. Sandra Rideout-Hanzak, Dr. David
Wester, and Dr. Mark Wallace for their help in putting this project and thesis together. A
special thanks to my family, particularly my parents, for supporting my efforts to follow
any path I choose. Thanks, also, to my field assistants, Nicole Cook and Roman Mayo,
for their help in collecting data for this project. Multiple graduate students at Texas Tech
helped me through this time as both friends and colleagues. Finally, multiple funding
agencies lent their support to complete all aspects of this project. The Natural Resource
Conservation Service, Texas Parks and Wildlife, and the Department of Natural
Resources Management at Texas Tech all provided financial support during these last
two and a half years, thank you.
i Texas Tech University, Anthony Roberts, May 2009
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ................................................................................................. i
TABLE OF CONTENTS .................................................................................................... ii
LIST OF TABLES ............................................................................................................. iii
CHAPTER
I. INTRODUCTION ........................................................................................................... 1
LITERATURE CITED ................................................................................................................ 6
II. GRASSLAND BIRD COMMUNITIES DURING THE BREEDING AND WINTER
SEASONS IN THE TEXAS PANHANDLE FOLLOWING A LARGE-SCALE
WILDFIRE ....................................................................................................................... 10
ABSTRACT............................................................................................................................... 10
INTRODUCTION ..................................................................................................................... 11
STUDY AREA .......................................................................................................................... 14
METHODS ................................................................................................................................ 16
RESULTS .................................................................................................................................. 19
DISCUSSION ............................................................................................................................ 23
MANAGEMENT IMPLICATIONS.......................................................................................... 27
LITERATURE CITED .............................................................................................................. 28
III. RESPONSE OF NESTING GRASSLAND BIRDS TO LARGE-SCALE WILDFIRE
IN THE TEXAS PANHANDLE ...................................................................................... 39
ABSTRACT............................................................................................................................... 39
INTRODUCTION ..................................................................................................................... 40
STUDY AREA .......................................................................................................................... 43
METHODS ................................................................................................................................ 44
RESULTS .................................................................................................................................. 47
DISCUSSION ............................................................................................................................ 54
MANAGEMENT IMPLICATIONS.......................................................................................... 56
LITERATURE CITED .............................................................................................................. 58
ii Texas Tech University, Anthony Roberts, May 2009
LIST OF TABLES
2.1. Avian community measures for breeding season birds among burned and unburned
plots associated with the East Amarillo Complex wildfires of 2006. ............................... 33
2.2. Avian abundance and percent composition (in parenthesis) on burned and unburned
shortgrass plots associated with the East Amarillo Complex wildfires of 2006............... 34
2.3. Matrix of similarity using Jaccard’s index (Cn) between years and burn conditions for
both shortgrass and mixedgrass sites associated with the East Amarillo Complex
wildfires of 2006. .............................................................................................................. 35
2.4. Densities (number per hectare ± SE) for the three most common species detected
during the breeding season on burned and unburned shortgrass plots associated with the
East Amarillo Complex wildfires of 2006. ....................................................................... 35
2.5. Winter avian abundance and percent composition (in parenthesis) on shortgrass plots
associated with the East Amarillo Complex wildfires of 2006......................................... 36
2.6. Avian abundance and percent composition (in parenthesis) on burned and unburned
mixedgrass plots associated with the East Amarillo Complex wildfires of 2006............. 37
2.7. Densities (number per hectare ± SE) for the three most common species detected
during the breeding season on burned and unburned mixedgrass plots associated with the
East Amarillo Complex wildfires of 2006. ....................................................................... 38
2.8. Winter avian abundance and percent composition (in parenthesis) on mixedgrass
plots associated with the East Amarillo Complex wildfires of 2006. ............................... 38
3.1. Species nesting abundance and percent composition (in parenthesis) on shortgrass
plots associated with the East Amarillo Complex wildfires of 2006. ............................... 62
3.2. Total passerine nest success using Mayfield’s estimate for both shortgrass and
mixedgrass plots associated with the East Amarillo Complex wildfires of 2006............. 63
3.3. Grasshopper sparrow nest site characteristics on burned and unburned shortgrass
prairie sites associated with the East Amarillo Complex wildfires of 2006. Means are for
nests on burned (n = 4) and unburned (n = 12) plots as well as all nests (n = 16) and all
random points (n = 16)...................................................................................................... 64
iii Texas Tech University, Anthony Roberts, May 2009
3.4. Lark sparrow nest site characteristics on burned and unburned shortgrass prairie sites
associated with the East Amarillo Complex wildfires of 2006. Means are for nests on
burned (n = 12) and unburned (n = 13) plots as well as all nests (n = 25) and all random
points (n = 25). .................................................................................................................. 65
3.5. Species nesting abundance and percent composition (in parenthesis) on mixedgrass
plots associated with the East Amarillo Complex wildfires of 2006. ............................... 66
3.6. Grasshopper sparrow nest site characteristics on burned and unburned mixedgrass
prairie sites associated with the East Amarillo Complex wildfires of 2006. Means are for
nests on burned (n = 15) and unburned (n = 4) plots as well as all nests (n = 19) and all
random points (n = 19)...................................................................................................... 67
3.7. Cassin’s sparrow nest site characteristics on burned and unburned mixedgrass prairie
sites associated with the East Amarillo Complex wildfires of 2006. Means are for nests
on burned (n = 18) and unburned (n = 8) plots as well as all nests (n = 26) and all random
points (n = 26). .................................................................................................................. 68
3.8. Lark sparrow nest site characteristics on burned and unburned mixedgrass prairie
sites associated with the East Amarillo Complex wildfires of 2006. Means are for nests
on burned (n = 10) and unburned (n = 4) plots as well as all nests (n = 14) and all random
points (n = 14). .................................................................................................................. 69 iv Texas Tech University, Anthony Roberts, May 2009
CHAPTER I
INTRODUCTION
On 12 March 2006 two large-scale wildfires were ignited east of Amarillo in the
Texas panhandle. Higher than normal rainfall in the spring of 2005 produced abundant
herbaceous growth and was followed by drought conditions during the subsequent
summer and fall. This produced a high amount of cured fuel, resulting in ideal conditions
for a large wildfire (Van Speybroeck et al. 2006). The two fires started approximately ten
minutes apart, around 11am, and are known as the Borger and I-40 fires. The Borger fire
occurred primarily in Hutchinson and Roberts counties and the I-40 fire burned primarily
in Gray and Wheeler counties. Together they burned over 360,000 ha of predominantly
private ranch lands in what is known as the East Amarillo Complex (EAC) (Zane et al.
2006). Human losses from these fires included twelve lives, 22 homes and over 2,500
head of livestock (Zane et al. 2006). Both fires were contained four days after ignition,
leaving more than 360,000 ha of bare earth during the historically windy spring season on
the Southern High Plains (Van Speybroeck et al. 2006). Widespread vegetation loss on
the mixedgrass and shortgrass ecosystems had potentially large negative impacts on
many species including lesser prairie-chicken (Tympanuchus pallidicinctus), mountain
plover (Chanadrius morinellus), and numerous species of grassland obligate songbirds
that breed in the Southern High Plains.
1 Texas Tech University, Anthony Roberts, May 2009
There have been numerous studies illustrating the decline of grassland birds over
the last sixty years, the most widely cited being Knopf (1994), who stated that this guild
has experienced the most consistent and widespread declines of any guild of birds in
North America. Among these are numerous high priority species in the Southern High
Plains such as mountain plover, long-billed curlew (Numenius americanus), Cassin’s
sparrow (Aimophila cassinii), and lark sparrow (Chondestes grammacus) (Johnsgard
1973, Knopf 1994). In addition, birds of the shortgrass prairies are some of the least
ecologically understood of all grassland birds (Askins et al. 2007). Multiple
anthropogenic forces have contributed to long-term declines in grassland bird
populations, especially habitat loss and fragmentation due to urban development,
agriculture, and infrastructure (Herkert 1995, Peterjohn and Sauer 1999). In the Texas
panhandle, where the EAC wildfires occurred, these anthropogenic forces have occurred
to a lesser degree than in other parts of the Great Plains. Contrary to fragmented areas of
the Great Plains, the problem in the Texas Panhandle may not be strictly linked to habitat
loss but instead could be linked to fire suppression or other altered ecological drivers.
Fire may be an important tool in restoring the native vegetation and microclimate
characteristics (Brockway et al. 2002), and ultimately increasing avian productivity in
grasslands. This is especially true in the Texas Panhandle where native rangeland is a
dominant part of the landscape and could potentially be a stronghold for grassland birds
on the Southern High Plains.
Previous studies of grassland bird response to fire have focused on the effects of
prescribed fire (e.g. Huber and Steuter 1984; Pylypec 1991; Madden et al. 1999;
Rohrbaugh et al. 1999; Vickery et al. 1999). Relatively few studies have assessed the
2 Texas Tech University, Anthony Roberts, May 2009
avian response following major wildfires in grasslands. Wildfires may be more intense
and cover larger spatial scales than prescribed fires, and therefore have a unique effect on
the avian community (Smith 2000). A grassland wildfire study in Arizona found a
decrease in litter cover drove the most individual species responses in burned areas,
though this study focused more on the suitability of introduced grasses after fire than
strictly population changes on native grasslands (Bock and Bock 1992). Populations of
individual species react differently to grassland management, including fire, with
different species preferring different specific habitat conditions (Madden et al. 1999). As
prairies proceed through succession following fire, species will react in varying ways.
Winter avian communities have different needs than do breeding season species, and fire
may likewise affect them in other ways. Unfortunately, few studies have examined
general grassland bird populations in the winter and none have looked at winter
populations following fire.
Avian community response to fire has been shown to follow patterns of individual
species responses to habitat alterations due to these fires. Changes in individual species
population changes as a result of habitat alteration usually occur as a result of general
habitat associations. For example, species that prefer tall, dense vegetation will be
negatively impacted by fire while those that prefer short, sparse vegetation will be
positively impacted by fire. Here I would like to examine the four most common species I
found during this research and their known habitat associations and relate that to impacts
of fire on these species.
Horned lark (Eremophila alpestris) is a species with Holarctic distribution that
prefers short, sparse vegetation, and occurs at its highest densities on landscapes where
3 Texas Tech University, Anthony Roberts, May 2009
this is the case, such as shortgrass prairies and agricultural lands (Beason 1995). They use
areas dominated by bareground and grasses no taller than a few centimeters (Wiens et al.
1987). Horned larks are often the first species to colonize bare areas after a disturbance or
habitat alteration such as fire or prairie restoration (Whitmore 1979). Winter habitat
associations are similar, as horned larks are found in the shortest vegetation on the
landscape (Grzybowski 1976, 1983). These habitat preferences suggest that fire has a
positive impact on populations of horned larks and they should be prevalent on areas
burned during the EAC, similar to what was found after wildfire in Arizona (Bock and
Bock 1992).
Lark sparrows use a wider variety of habitats than the horned lark. They are found
in riparian areas and most grassland types, though they prefer sparse litter and a
landscape with a shrub component (Wiens and Rotenberry 1981). Relatively little is
known about this species nesting ecology (Lusk et al. 2003), though nests have been
found both on the ground and in shrubs or small trees, up to 4 m (McNair 1985). Past
research has shown that burning is beneficial to lark sparrow populations. Research in
Texas (Renwald 1977), Kansas (Robel et al. 1998), and Arizona (Bock and Bock 1992)
have all shown a short-term increase in populations on burned areas for this species.
These areas have similar habitat conditions as the Texas Panhandle, suggesting that the
EAC should have similar effects on populations of lark sparrows.
Grasshopper sparrow (Ammodramus savannarum) is a widespread grassland bird.
Habitat preferences for this species vary on the type of grassland. For example, in the
eastern United States and the northern Great Plains, grasshopper sparrows occupy areas
of sparse vegetation, while in the arid grasslands of the southwest United States, such as
4 Texas Tech University, Anthony Roberts, May 2009
that found in the Texas panhandle, this species occupies more dense vegetation, often
with a shrub component (Vickery 1996). They are also more likely to occupy large,
continuous habitat than small fragments (Herkert 1994). In the southwest, where
grasshopper sparrows prefer dense vegetation, densities generally decrease following fire.
This pattern was followed in Bock and Bock’s (1992) study of wildfire in Arizona.
Cassin’s sparrow is found on the arid grasslands of the Southern High Plains.
Their seasonal movements, migratory status, and other population traits are unusual for
many grassland songbirds and there has been a shortage of research on this species
compared to many others. One of the reasons for the lack of research may be due to large
annual population fluctuations as a result of summer rainfall (Dunning et al. 1999). Males
use shrubs or potentially bare earth as flight perches to perform elaborate courtship
displays. Bock and Bock (1992) report that this species temporarily declined after
wildfire, possibly because of lack of thick vegetation for nesting sites.
The EAC offers a unique opportunity to fill gaps in our knowledge of the avian
community in the Southern High Plains and its response to wildfires. Saab and Powell
(2005) reviewed current knowledge of fire and avian ecology in North America but had
no information on fires of the shortgrass and mixedgrass prairie communities. Much of
the work on grassland birds and fire has focused on tallgrass prairies and northern
mixedgrass prairies, often examining avian communities on public lands. This area of the
Southern High Plains is dominated by private ranchland used extensively for grazing.
Comparatively little is known about how birds on the Southern High Plains react to fire,
whether wild or prescribed. It is thought that fire was historically the principle ecological
driver on tallgrass prairies. This contrasts with shortgrass prairies where drought and
5 Texas Tech University, Anthony Roberts, May 2009
grazing were probably more important than fire (Milchunas et al. 2008), making the
effects of fire in this area much different than those seen in tallgrass systems (Askins et
al. 2007). The continuing cycle of bison (Bos bison) grazing and fire, as described by
Fuhlendorf and Engel (2001), may have greatly affected grassland birds historically, and
the EAC gives us an opportunity to examine a piece of that relationship. To better
understand the benefits of fire on this landscape I studied the grassland bird community
on areas affected by the EAC during the second and third years after the fires. I surveyed
populations during the breeding and the winter seasons, and examined species diversity,
composition, and densities. In addition I studied grassland bird nesting ecology on areas
affected by the EAC. I searched for and monitored nests on shortgrass and mixedgrass
prairie sites to provide a thorough account of nesting ecology in the Southern High Plains
and how it changes in response to wildfire.
This thesis is written following Journal of Wildlife Management style where it
doesn’t conflict with Texas Tech University guidelines. Chapter 1 includes a summary of
my research and background material that may be useful to readers. Each of the next two
chapters is written as a stand alone document intended for publication.
LITERATURE CITED
Askins, R.A., F. Chavez-Ramirez, B.C. Dale, C.A. Hass, J.R. Herkert, F.L. Knopf, and
P.D. Vickery. 2007. Conservation of grassland birds in North America:
Understanding ecological processes in different regions. Ornithological
Monographs 64:1-46.
6 Texas Tech University, Anthony Roberts, May 2009
Beason, R.C. 1995. Horned Lark (Eremophila alpestris). In A. Poole and F. Gill editors.
The Birds of North America #195. Academy of Natural Sciences, Washington,
D.C., USA.
Bock, C.E and J.H. Bock. 1992. Response of birds to wildfire in native versus exotic
Arizona grassland. The Southwestern Naturalist 37:73-81.
Brockway, D.G., R.G. Gateway, and R.B. Paris. 2002. Restoring fire as an ecological
process in shortgrass prairie ecosystems: initial effects of prescribed burning
during the dormant and growing seasons. Journal of Environmental Management
65:135-152.
Grzybowski, J.A. 1976. Habitat selection among some grassland birds wintering in
Oklahoma. Annuals of the Oklahoma Academy of Science 1976:176-182.
Grzybowski, J.A. 1983. Patterns of space use in grassland bird communities during
winter. Wilson Bulletin 95:590-601.
Fuhlendorf, S.D., and D.M. Engle. 2001. Restoring heterogeneity on rangelands:
ecosystem management based on evolutionary grazing patterns. BioScience
51:625-632.
Herkert, J.R. 1994. The effects of habitat fragmentation on Midwestern grassland bird
communities. Journal of Ecological Applications 4:461-471.
Herkert, J.R. 1995. An analysis of Midwestern breeding bird population trends: 19661993. American Midland Naturalist 134:41-50.
Huber, G.E., and A.A. Steuter. 1984. Vegetation profile and grassland bird response to
spring burning. Prairie Naturalist 16:55-61.
Johnsgard, P.A. 1973. Birds of the Great Plains: Breeding species and their distribution.
University of Nebraska Press, Lincoln, Nebraska, USA.
Knopf, F.L. 1994. Avian assemblages on altered grasslands. Studies in Avian Biology
15:247-257.
7 Texas Tech University, Anthony Roberts, May 2009
Lusk, J.J., K. Suedkamp-Wells, F.S. Guthery, and S.D. Fuhlendorf. 2003. Lark sparrow
(Chondestes grammacus) nest-site selection and success in a mixed-grass prairie.
Auk 120:120-129.
Madden, E.M., A.J. Hansen, and R.K. Murphy. 1999. Influence of prescribed fire history
on habitat and abundance of passerine birds in northern mixed-grass prairie.
Canadian Field-Naturalist 113:627-640.
McNair, D.B. 1985. A comparison of oology and nest record card data in evaluating the
reproductive biology of lark sparrows (Chondestes grammacus). Southwestern
Naturalist 30:213-224.
Milchunas, D.G., W.K. Lauenroth, I.C. Burke, and J.K. Detling. 2008. Effects of grazing
on vegetation. In W.K. Lauenroth and I.C. Burke editors. Ecology of the
Shortgrass Steppe. Oxford University Press, Oxford, UK.
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.
Pylypec, B. 1991. Impacts of fire on bird populations in a fescue prairie. Canadian FieldNaturalist 105:346-349.
Renwald, J.D. 1977. Effect of fire on lark sparrow nesting densities. Journal of Range
Management 30:283-285.
Robel, R.J., J.P. Hughes, S.D. Hull, K.E. Kemp, and D.S. Klute. 1998. Spring burning:
resulting avian abundance and nesting in Kansas CRP. Journal of Range
Management 51: 132-138.
Rohrbaugh Jr., R.W., D.L. Reinking, D.H. Wolfe, S.K. Sherrod, and M.A. Jenkins. 1999.
Effects of prescribed burning and grazing on nesting and reproductive success of
three grassland passerine species in tallgrass prairie. Studies in Avian Biology
19:165-170.
Saab, V. A., and H. D. W. Powell, editors. 2005. Fire and avian ecology in North
America. Studies in Avian Biology, Publication of the Cooper Ornithological
Society.
8 Texas Tech University, Anthony Roberts, May 2009
Smith, J.K., editor. 2000. Wildland fire in ecosystems: effects of fire on fauna. General
Technical Report RMRS-GTR-42. U.S. Department of Agriculture, Forest
Service, Rocky Mountain Research Station, Ogden, Utah, USA.
Van Speybroeck, K.M., A.R. Patrick, and M.C. Oaks. 2006. Climate variability and the
Texas fire weather season of 2005-2006: an historic perspective of a statewide
disaster. AMS forum: Climate change manifested by changes in weather. 19th
conference on climate variability and change.
Vickery, P.D. 1996. Grasshopper Sparrow (Ammodramus savannarum). In A. Poole and
F. Gill editors. The Birds of North America #293. Academy of Natural Sciences,
Washington, D.C., USA.
Vickery, P.D., M.L. Hunter, and J.V. Wells. 1999. Effects of fire and herbicide treatment
on habitat selection in grassland birds in southern Maine. Studies in Avian
Biology 19:149-159.
Whitmore, R.C. 1979. Short-term change in vegetation structure and its effects on
grasshopper sparrows in West Virginia. Auk 96:621-625.
Wiens, J.A. and J.T. Rotenberry. 1981. Habitat associations and community structure in
shrubsteppe environments. Ecological Monographs 51:21-41.
Wiens, J.A., J.T. Rotenberry, and B. van Horne. 1986. Habitat occupancy patterns on
North American shrubsteppe birds: the effect of spatial scale. Oikos 48:132-147.
Zane, D., J. Henry, C. Lindley, P.W. Pendergrass, D. Galloway, T. Spencer, and M.
Stanford. 2006. Surveillance of mortality during the Texas panhandle wildfires
(March 2006). www.wildfirelessons.net.
9 Texas Tech University, Anthony Roberts, May 2009
CHAPTER II
GRASSLAND BIRD COMMUNITIES DURING THE BREEDING AND WINTER
SEASONS IN THE TEXAS PANHANDLE FOLLOWING A LARGE-SCALE
WILDFIRE
ABSTRACT
Two large-scale wildfires occurred in the spring of 2006 in the Texas Panhandle
and burned over 360,000 ha of predominantly private ranchland. These fires are now
known as the East Amarillo Complex (EAC) wildfires. There have been few studies of
the effects of wildfire on prairie communities, particularly on declining populations of
grassland birds. I studied the wintering and breeding season communities of grassland
birds on shortgrass and mixedgrass prairie sites during the second and third years
following these wildfires. I examined multiple community composition parameters and
looked closely at densities of individual species using Program DISTANCE. On
shortgrass sites, species that prefer sparse vegetation and bareground, such as horned lark
(Eremophila alpestris), seemed to benefit from the fires, while others, such as
grasshopper sparrow (Ammodramus savannarum), that prefer more dense vegetation,
appeared to have densities that were lower than unburned areas. Mixedgrass sites also
exhibited species specific shifts due to changes in vegetation in 2007, two breeding
seasons after the fires, though by 2008 grassland bird communities on burned plots were
10 Texas Tech University, Anthony Roberts, May 2009
similar to those on unburned plots. Winter grassland bird populations showed few
changes after the fires, though this may be a result of small sample sizes. Population
measures collected in my study suggest that avian communities have likely returned to
pre-burn levels within three years following the EAC wildfires. The persistence of a
diverse and abundant avian community in this area relies on continuing disturbances such
as wildfire or prescribed fire, grazing, and drought. Though the EAC wildfires affected
people living in this area tremendously, they appeared to have few if any short-term
negative impacts on the avian community. In fact, they were more than likely beneficial
in providing similar services as historic fire regimes on the Southern High Plains.
INTRODUCTION
On 12 March 2006 two large-scale wildfires were ignited east of Amarillo in the
Texas panhandle. Higher than normal rainfall in the spring of 2005 produced abundant
herbaceous growth and was followed by drought conditions during the subsequent
summer and fall. This produced a high fine fuel load, resulting in ideal conditions for a
large wildfire (Van Speybroeck et al. 2006). The two fires started approximately ten
minutes apart, around 11 am, and are known as the Borger and I-40 fires. The Borger fire
occurred primarily in Hutchinson and Roberts counties and the I-40 fire burned primarily
in Gray and Wheeler counties. Together they burned over 360,000 ha of predominantly
private ranch lands in what is known as the East Amarillo Complex (EAC) (Zane et al.
2006). Casualties from these fires included twelve lives, 22 homes and over 2,500 head
of livestock (Zane et al. 2006). Both fires were contained four days after ignition, leaving
11 Texas Tech University, Anthony Roberts, May 2009
more than 360,000 ha of bare earth during the historically windy spring season on the
Southern High Plains (Van Speybroeck et al. 2006). Widespread vegetation loss on the
mixedgrass and shortgrass ecosystems had potentially large negative impacts on many
species including lesser prairie-chicken (Tympanuchus pallidicinctus), mountain plover
(Chanadrius morinellus), and numerous species of grassland obligate songbirds that
breed in the Southern High Plains.
There have been numerous studies illustrating the decline of grassland birds over
the last sixty years, the most widely cited being Knopf (1994), who stated that this guild
has experienced the most consistent and widespread declines of any guild of birds in
North America. Among these are numerous high priority species in the Southern High
Plains such as mountain plover, long-billed curlew (Numenius americanus), Cassin’s
sparrow (Aimophila cassinii), and lark sparrow (Chondestes grammacus) (Johnsgard
1973, Knopf 1994). In addition, birds of the shortgrass prairies are some of the least
ecologically understood of all grassland birds (Askins et al. 2007). Multiple
anthropogenic forces have contributed to long-term declines in grassland bird
populations, especially habitat loss and fragmentation due to urban development,
agriculture, and infrastructure (Herkert 1995, Peterjohn and Sauer 1999). In the Texas
panhandle, where the EAC wildfires occurred, these anthropogenic forces have occurred
to a lesser degree than in other parts of the Great Plains. Contrary to fragmented areas of
the Great Plains, the problem in the Texas Panhandle may not be strictly linked to habitat
loss but instead could be linked to fire suppression or other altered ecological drivers.
Fire may be an important tool in restoring the native vegetation and microclimate
characteristics (Brockway et al. 2002), and ultimately increasing avian productivity in
12 Texas Tech University, Anthony Roberts, May 2009
grasslands. This is especially true in the Texas Panhandle where native rangeland is a
dominant part of the landscape and could potentially be a stronghold for grassland birds
on the Southern High Plains.
Previous studies of grassland bird response to fire have focused on the effects of
prescribed fire (e.g. Huber and Steuter 1984; Pylypec 1991; Madden et al. 1999;
Rohrbaugh et al. 1999; Vickery et al. 1999). Relatively few studies have assessed the
avian response following major wildfires in grasslands. Wildfires may be more intense
and cover larger spatial scales than prescribed fires, and therefore have a unique effect on
the avian community (Smith 2000). A grassland wildfire study in Arizona found a
decrease in litter cover drove the most individual species responses in burned areas,
though this study focused more on the suitability of introduced grasses after fire than
strictly population changes on native grasslands (Bock and Bock 1992). Populations of
individual species react differently to grassland management, including fire, with various
species preferring specific habitat conditions (Madden et al. 1999). As prairies proceed
through succession following fire, species will react in varying ways. Winter avian
communities have different needs than do breeding season species, and fire may likewise
affect them in other ways. Unfortunately, few studies have examined general grassland
bird populations in the winter and none have looked at winter populations following fire.
The EAC presented a unique opportunity to study the effects of large-scale
wildfires on grassland bird populations. Much of the work on grassland birds and fire has
focused on tallgrass prairies and northern mixed-grass prairies, where climate and rainfall
differs from the Texas Panhandle. Comparatively little is known about how birds on the
Southern High Plains react to fire, whether wild or prescribed. It is thought that fire was
13 Texas Tech University, Anthony Roberts, May 2009
historically the principle ecological driver in tallgrass prairies. In contrast, drought and
grazing were probably more important than fire in the shortgrass prairies (Milchunas et
al. 1988), making the effects of fire much different than those seen in tallgrass systems
(Askins et al. 2007). The continuing cycle of bison (Bos bison) grazing and fire, as
described by Fuhlendorf and Engel (2001), may have greatly affected avian communities
historically and the EAC gives us an opportunity to examine a piece of that relationship.
The Texas panhandle is unique in that it contains large tracts of relatively undisturbed
rangeland, a condition not seen in most parts of the Great Plains. To better understand the
benefits of fire on this landscape I studied the grassland bird community on areas affected
by the EAC during the second and third years following the fires. I surveyed populations
during the breeding and the winter seasons. There were no studies being conducted in this
area prior to March of 2006 so I compared populations on burned sites with populations
on similar, unburned sites, close to burned areas.
STUDY AREA
I surveyed grassland birds on private ranches in Roberts, Gray and Donley
counties in the Texas panhandle. These counties lay in a transition zone between
shortgrass and mixedgrass prairie types in the Southern High Plains in an area known as
the Llano Estacado. The Llano Estacado occurs in Texas and New Mexico and is
bordered by the Canadian River to the north and the Caprock Escarpment in the south
and east. The landscape is characterized by rolling hills leading to flat plains and is
interspersed with ephemeral wetland depressions known as playas (Williams and Welker
14 Texas Tech University, Anthony Roberts, May 2009
1966, Wyrick 1981). Elevation ranges from 300 m to over 900 m (Wyrick 1981). Along
the edge of the flat plains and rolling hills there are often steep bluffs dissected by
canyons and drainage channels that contain different vegetation communities due to
changes in aspect, water availability, and soil types (Williams and Welker 1966).
Climate in this area is characterized by hot summers and cold to mild winters
(Wyrick 1981). Mean summer temperature is 25 C and mean winter temperature is 2 C
but temperatures fluctuate extensively within seasons (Williams and Welker 1966).
Western short-grass prairie receives markedly less precipitation than the eastern Great
Plains and over 80% of moisture occurs between April and September (Williams and
Crump 1980). Rainfall typically occurs in the form of thunderstorms, and precipitation
may be absent for weeks or months during the summer, causing drought conditions
(Borchert 1950), though on average this area receives 53 cm of precipitation a year
(Williams and Welker 1966).
Pre-European settlement vegetation of this area was mainly blue grama
(Bouteloua gracilis) and sideoats grama (Bouteloua curtipendula) with low lands and
sandy soils containing more little bluestem (Schizachyrium scoparium), switchgrass
(Panicum virgatum), and indiangrass (Sorghastrum nutans), but agriculture has converted
a large portion of the native prairie into monoculture croplands (Wyrick 1981). Shrubs
include mesquite (Prosopis glandulosa) and red-berry juniper (Juniperis pinchotii) in
tight upland soils and sand-shinnery oak (Quercus havardii) and sand sagebrush
(Artemesia filifolia) in less dense, sandier soils. The EAC impacted two bird conservation
regions, the shortgrass prairie (BCR 18) and the central mixed-grass prairie (BCR 19)
(USFWS 2000).
15 Texas Tech University, Anthony Roberts, May 2009
METHODS
Study Plot Selection
Selection of the twenty plots used in this study was based on access to private
property, burn history, and vegetation community type of either shortgrass or mixedgrass
prairie. The latter distinction was made by examining soil type and dominant vegetation.
Burn history was established by talking with landowners and local officials and looking
for fire scars on any woody vegetation in the area such as prickly pear cactus (Opuntia
spp), catclaw mimosa (Mimosa aculeaticarpa), or sand sagebrush. The twenty plots were
equally distributed among shortgrass and mixedgrass, burned and unburned areas. This
resulted in a study design with five replicates within each of four treatments, burned
shortgrass, unburned shortgrass, burned mixedgrass, and unburned mixedgrass.
Individual plots were placed at least 1 km apart and 0.5 km from the known fire
boundary, roads, or other vegetation or topology changes.
Breeding Season Surveys
I used fixed-radius point counts to survey breeding season avian populations
across the twenty plots. I conducted surveys using a three-by-three grid of nine 75 m
radius point counts for a total survey area of nearly 16 ha. Surveys were performed twice
during the 2007 and 2008 breeding seasons, two and three years after the EAC,
respectively (Reynolds et al. 1980, Hutto et al. 1986). Throughout the study I used three
observers to conduct point counts. All observers had either prior experience with
16 Texas Tech University, Anthony Roberts, May 2009
grassland bird identification or I trained them extensively prior to surveys. Point count
centers were placed 200 m apart to minimize risk of recounting individuals and all points
were 75 m from field edges and vegetation changes (Ralph et al. 1993). I conducted
surveys between a half hour before and three hours after sunrise. To reduce potential
weather-associated bias in detection I did not survey points during inclement weather
such as rain (Ralph et al. 1993). All birds heard or seen in the 75 m radii within a 7-min
window were recorded and the distance from the observer was estimated. Birds flying
overhead but not utilizing the area within the 75 m radii were not recorded.
Winter Season Surveys
Due to the gregarious behavior and inactivity of birds during the winter, point
counts were not a viable option for winter season surveys in this area. Therefore, I used
fixed-width transect counts for winter season bird populations (Emlen 1977). This
consisted of five parallel transects each 400 m long and 50 m wide and provided for a
survey area of 10 ha at each study plot. Each study plot that was used for breeding season
surveys was also used for winter season surveys. I completed surveys at each site once
each winter, in February of each year. Transect width was kept to 50 m to increase the
possibility of seeing all birds in the area, and facilitate accurate distance measurements
between transects and the bird (Buckland et al. 2001, Roberts and Schnell 2006).
Transects were placed 75 m apart to decrease likelihood of double counting (Delisle and
Savidge 1997). I conducted all winter transect counts using two observers, both
extensively trained in grassland bird identification. I conducted surveys from a half hour
17 Texas Tech University, Anthony Roberts, May 2009
after sunrise to an hour before sunset and recorded all individuals and their estimated
distances from the transect (Burnham et al. 1980).
Data Analysis
For abundance and species diversity measures I used both the May and June
surveys to create an average abundance over the breeding season. I calculated species
diversity using the Shannon diversity index (H´) as well as Simpson’s diversity index
(D). Though Taylor (1978) suggested that diversities calculated over a variety of samples
are normally distributed, I used the more robust t-test suggested by Hutcheson (1970) to
compare H´ across conditions. I derived evenness (E) from H’ using a ratio of observed
to maximum diversity as described by Magurran (1988). To calculate similarity among
burn treatments and years I used the Jaccard index (Cn) (Janson and Vegelius 1981).
I used Program DISTANCE to calculate individual species densities (Buckland et
al. 1993). This program allows for individual density estimates based on varying species
detection rates. Model selection for detection rates was based on suggestions from
Buckland et al. (2001). I only used singing males from the June surveys to calculate
density estimates. May surveys included migrants that may have affected territory size
and placement, and hence, densities of individual species. To compare density estimates
among years and burn treatments I used the t-test recommended by Buckland et al.
(2001). The degrees of freedom used in this test are an estimate and result in p-values that
are approximate only (Buckland et al. 2001). An alpha level of 0.05 was used in all cases.
I made no attempt to compare densities between shortgrass and mixedgrass sites.
18 Texas Tech University, Anthony Roberts, May 2009
RESULTS
Shortgrass
Species richness and diversity.— In 2007, I detected 13 species on shortgrass plots, 11 of
which occurred on burned plots and 12 on unburned plots (Table 2.1). Where present,
lark sparrow was abundant and was the only species seen strictly on burned plots. Cliff
swallow (Petrochelidon pyrrhonota) and scaled quail (Callipepla squamata) were seen
strictly on unburned plots with one and two individuals seen per species, respectively
(Table 2.2). Fourteen species were detected in 2008 on burned plots and 10 of those were
detected on unburned plots (Table 2.1). Cliff swallow, barn swallow (Hirundo rustica),
common grackle (Quiscalus quiscula), and northern bobwhite (Colinus virginianus) were
all detected on burned plots and not unburned plots, but with one or two individuals
representing each species (Table 2.2). Dickcissels (Spiza Americana) were detected on
both burned and unburned areas in 2007 but went undetected in 2008. Similarity
measures (Cn) suggest there was more variation in species between years than within
years on the same plots (Table 2.3).
Grasshopper sparrow (Ammodramus savannarum), lark sparrow, horned lark
(Eremophila alpestris), and western meadowlark (Sturnella neglecta) represented
between 70% and 85% of all individuals detected across shortgrass plots (Table 2.2).
Western meadowlarks were the most abundant species on unburned plots in both years
whereas grasshopper sparrows were the most abundant species on burned plots both
years.
19 Texas Tech University, Anthony Roberts, May 2009
Shannon diversity was significantly higher on unburned areas in 2007 (t = 2.06, df
= 239, P < 0.05) but diversity was similar across burn conditions in 2008 (Table 2.1).
Diversity in burned areas was significantly lower in 2007 than in 2008 (t = 4.99, df =
223, P < 0.05) but was not different between years in unburned plots. Measurements of
evenness suggest that species were less equitably distributed within plots in 2007 than in
2008 and more evenly distributed among unburned areas than burned plots (Table 2.1).
Density of species.— There were only sufficient observations to perform density analysis
for grasshopper sparrow, horned lark and western meadowlark. On average, grasshopper
sparrows had the highest density of all species recorded during point count surveys. In
2007 there were an estimated 0.49 and 0.56 grasshopper sparrows per hectare in burned
and unburned plots, respectively (Table 2.4). In 2008 densities were 0.38/ha and 0.39/ha
among burned and unburned plots, respectively. Differences between burn conditions or
between years were not significant.
Estimated densities of horned larks were 0.32/ha on burned plots and 0.41/ha on
unburned plots in 2007 (Table 2.4). Burn plots were not significantly different between
2007 (0.32/ha) and 2008 (0.32/ha). However, densities were substantially, though not
significantly, lower on unburned plots (0.14/ha) in 2008. Estimated density of western
meadowlarks in 2007 were higher on unburned plots (0.58/ha) compared to burned plots
(0.36/ha) (t = 2.4, df = 66.6, P < 0.05). In 2008, densities of western meadowlarks
decreased on unburned plots (t = 2.2, df = 66.9, P < 0.05) to 0.38/ha (Table 2.4).
20 Texas Tech University, Anthony Roberts, May 2009
Winter abundance.— I detected very few individuals during the winter surveys, possibly
due to gregarious behavior of individuals and feeding in nearby grain fields, rather than
in native prairie that I was surveying. I detected no birds on the three shortgrass plots I
surveyed in 2007. In 2008 I surveyed ten plots and detected 36 individual birds
representing four species (Table 2.5). Due to the secretive nature of these birds during
winter months, I failed to identify some individuals to species (n = 5). This consisted of
unknown sparrows (n=2) and unknown longspurs (n=3). The most abundant species on
burned sites was horned lark (n = 15), and the most abundant species on unburned sites
was western meadowlark (n = 8) (Table 2.5). Other species I detected were scaled quail
(n=2) and McCown’s longspur (Calcarius mccownii) (n=2).
Mixedgrass
Species richness and diversity.— I observed twelve species on mixedgrass sites in 2007
during the breeding season. Nine species were observed on burned areas and 11 on
unburned areas (Table 2.1). Barn swallow, blue grosbeak (Guiraca caerulea), and
mourning dove (Zenaida macroura) were present in small numbers strictly on unburned
sites (Table 2.6). Scissor-tailed flycatcher (Tyrannus forticatus) was the only species seen
strictly on burned areas in 2007. In 2008, I detected17 species on mixedgrass plots. Barn
swallow, eastern kingbird (Tyrannus tyrannus), killdeer (Charadrius vociferous), and
lesser prairie-chicken were all seen in low numbers on burned areas but were not detected
on unburned sites (Table 2.6). Bullock’s oriole (Icterus bullockii), cliff swallow, eastern
meadowlark (Sturnella magna), and horned lark were only detected on unburned areas,
21 Texas Tech University, Anthony Roberts, May 2009
all in low numbers (Table 2.6). Dickcissels were detected in 2007 but went undetected in
2008.
Cassin’s sparrow, grasshopper sparrow, lark sparrow, and western meadowlark
were the dominant species detected across burn conditions and years, representing 80%
of all individuals (Table 2.6). Although grasshopper sparrows were frequently detected
both years on both burned and unburned plots, they were less common on burned sites in
2007. Western meadowlarks were nearly absent on unburned areas in 2008.
There was no difference in Shannon diversity across burn conditions in 2007. In
2008, diversity on unburned mixedgrass plots increased from that in 2007 (t = 3.45, df =
268, P < 0.05) and was significantly higher than diversity among burned plots (t = 2.59,
df = 235, P < 0.05) (Table 2.1). There was no change detected in diversity between years
on burned plots. Evenness measures suggest that all plots had similar species
distributions across conditions and years (Table 2.1).
Density of species.— Among mixedgrass plots I only obtained enough detections to
perform density analysis on western meadowlark, Cassin’s sparrow, and grasshopper
sparrow. In 2007, western meadowlarks had slightly higher densities on unburned sites
(0.40/ha) compared to burned sites (0.30/ha) (Table 2.7). In 2008 there was no statistical
significance in western meadowlark densities between burned (0.32/ha) and unburned
(0.34/ha) areas.
Observed densities of Cassin’s sparrows in 2007 were higher on unburned
(0.40/ha) compared to burned areas (0.28/ha) (Table 2.7), though densities were not
22 Texas Tech University, Anthony Roberts, May 2009
statistically significant. By 2008 densities on burned plots increased and decreased on
unburned plots such that density of Cassin’s sparrows were nearly identical on both plot
types (0.33/ha).
Grasshopper sparrows had significantly higher densities on unburned plots
(0.49/ha) than burned plots (0.13/ha) in 2007 (t = 4.3, df = 80.3, P < 0.05) (Table 2.7). In
2008, densities had increased on burned plots (0.47/ha) (t = 3.1, df = 80.1, P < 0.05) and
had changed little on unburned plots (0.38/ha) (Table 2.7).
Winter abundance.— Similar to shortgrass plots, I detected few individual birds while
completing winter surveys. In 2007 I surveyed seven plots and encountered 23
individuals representing four species (Table 2.8). While surveying ten plots in 2008 I
detected 19 individuals representing three species. Vesper sparrow (Pooecetes
gramineus) and horned lark were detected in 2007 but not in 2008. Chestnut-collared
longspur (Calcarius ornatus) was identified in 2008, but went unidentified in 2007.
Longspurs (Calcarius spp.) were the most prevalent species detected during both years
on burned and unburned plots (Table 2.8). Western meadowlark was also detected on
mixedgrass plots during winter surveys.
DISCUSSION
Historically, fire was a major ecological driver in both shortgrass and mixedgrass
ecosystems until intense livestock grazing and fire suppression altered vegetation and
fuels such that fires could not burn with historic frequency or intensity (Wright and
23 Texas Tech University, Anthony Roberts, May 2009
Bailey 1982). Some areas of the Great Plains have seen fire return to the landscape in the
form of prescribed fire as a management tool. The EAC wildfires occurred under vastly
different conditions than prescribed fire and thus had the potential to affect the avian
community in different ways. This study offers new insight about the effect of prairie
wildfires on grassland birds. Butcher and Niven (2007) presented an analysis of Breeding
Bird Survey and Christmas Bird Count data to examine the trend of North American
birds and listed the twenty most common species in the steepest decline. Nine of these
birds occur regularly in the Texas panhandle. Three of the most common species I
detected on shortgrass plots during my surveys are included on that list; grasshopper
sparrow, lark sparrow and horned lark. Due to the abundance of these species on areas
affected by the EAC wildfires, this area may provide important habitat for the continued
persistence of these species.
Across the study area there was above average rainfall in 2007, a year after the
fires. Though the Texas panhandle receives on average 53 cm of precipitation a year, in
2007 75 cm of precipitation fell, including 38 cm before breeding season started in May
(West Texas Mesonet 2007). This probably contributed to increased plant growth and
affected diversity and densities across the study area. During 2007 avian diversity
decreased on burned shortgrass plots but was similar to unburned areas by 2008, three
breeding seasons post-fire. The EAC wildfires were intense and left a more homogeneous
landscape where they passed. This may have offered fewer niches for grassland birds to
occupy compared to heterogeneous, unburned areas that were marked by differing
management regimes and, potentially, offered a wider variety of niches. This may have
explained the comparatively higher diversity seen among unburned plots in 2007. This is
24 Texas Tech University, Anthony Roberts, May 2009
substantiated by the increased evenness across plots in the landscape in 2008, which may
be indicative of more species and individuals moving into the created niches on burned
areas following increased vegetation growth.
On shortgrass prairie, grasshopper sparrows were the most abundant species, but
unburned plots had significantly higher densities than burned plots in 2007. This may be
a result of fewer quality nesting or foraging sites, or a decrease in available food
resources following the fires. However, densities were similar across burn conditions
three years after the EAC wildfires. Grasshopper sparrows have shown this trend in other
parts of their range as well. In Montana, they were significantly more abundant on
unburned plots after a wildfire (Bock and Bock 1987). Bock and Bock (1992) also
noticed lower numbers of grasshopper sparrows on burned areas for two years following
wildfire in Arizona.
Horned lark densities were similar across burn treatments in 2007, but by 2008
densities of this species had declined on unburned plots while staying steady on burned
plots. This is consistent with horned lark’s general habitat association, as they prefer
areas with high percentage of bare ground (Beason 1995). However, this does appear to
run counter to the speculation that vegetation recovery by 2008 led to increased
grasshopper sparrow density among burned plots. Western meadowlarks responded to the
wildfires consistent with general habitat preferences. In both years they were more
abundant on unburned plots which offered males more suitable perches for singing.
Two breeding seasons after the wildfires, in 2007, mixedgrass plots had similar
avian diversity and evenness on burned and unburned plots. Despite these measures, the
25 Texas Tech University, Anthony Roberts, May 2009
composition of the avian community differed between burn conditions. Multiple species
occurred on unburned plots that were not seen on burned plots. The number of
individuals seen on unburned plots in 2007 was also much higher than those counted on
burned areas (Table 2.6). This is demonstrated by densities of the most common species
seen during breeding season surveys in 2007, Cassin’s sparrow, grasshopper sparrow, and
western meadowlark. Densities of these species were similar between burn conditions by
2008, suggesting a return of the avian community to pre-burn levels. This same pattern of
a temporary shift following wildfire was seen in Arizona, though the timeline was
different, probably due to differing precipitation patterns (Bock and Bock 1992).
As mentioned before, grasshopper sparrows have been shown to be one of the
most common grassland species in North America that are also exhibiting the steepest
declines. In this study, grasshopper sparrows were present in higher densities on
unburned areas in 2007, but by 2008, three growing seasons after the fires, their densities
were similar across burn conditions, or even slightly higher on those areas that had been
burned. Cassin’s sparrows and western meadowlarks demonstrated similar patterns,
suggesting that three breeding seasons after the fires burned areas provided conditions
similar to unburned areas in terms of breeding and foraging habitat.
Population measures collected in my study suggest that avian communities have
likely returned to pre-burn population levels within the affected area three years
following the EAC wildfires, possibly as a result of increased vegetation growth due to
higher than average rainfall. This would follow a similar timeline presented by
Launchbaugh (1964) for plants. His paper stated that vegetation recovery from dry season
fire on the shortgrass prairie typically requires no longer than three years (Launchbaugh
26 Texas Tech University, Anthony Roberts, May 2009
1964). If vegetation had recovered by 2008, then the grassland birds that depend on the
plant community had a better chance to return to pre-burn breeding densities. In the
current study, there seemed to be a temporary shift in avian community composition after
the fires following species specific shifts, depending on life-history traits and ecosystem
preferences. Though these fires had tremendous affects on the people living in this area,
as of three years post fire they had few if any negative impacts on the avian community.
In fact, they were more than likely beneficial in providing similar services to plants and
soil as historic fire regimes on the Southern High Plains. Wildfire was a historic
occurrence throughout this landscape and the species that reside there have evolved lifehistory strategies to survive and thrive in a landscape shaped by fire and other
disturbances.
MANAGEMENT IMPLICATIONS
Little is known about how wildfires affected grassland systems historically. It is
thought that the ecological drivers of drought, grazing, and fire provided a continuum of
prairie succession that created a mosaic of vegetation stands across the grasslands (Ryan
1990; Fuhlendorf and Engel 2001). On areas affected by the EAC, the avian community
seems to be similar to that on unburned areas three years following the fires. This is not
necessarily a good thing, as the landscape may offer little in terms of diverse
microhabitats with a variety of successional stages. The grassland bird community may
be better served by increased disturbance in this area.
27 Texas Tech University, Anthony Roberts, May 2009
The persistence of a diverse and abundant avian community relies on continuing
disturbances such as wildfire or prescribed fire, grazing, and drought. Management of
grazing in the Texas panhandle varies based on landowner’s management goals and
ranching philosophy, and periodic drought is uncontrollable. What still needs to be
altered to produce historic ecosystem services in this area is fire. Wildfire is potentially
harmful to human lives and property and therefore is largely unsuitable as a management
option. Instead, prescribed fire has been used to mimic wildfire effects and reduce
wildfire potential (Pattison 1998). The effectiveness of fire surrogates such as mowing in
mimicking fire effects has not been researched on the Great Plains. Though mowing
would provide similar vegetation structure as fire, it would probably fail to reduce litter
cover or return nutrients to the soil and temporarily increase vegetation growth.
Fortunately, landowner’s attitudes towards prescribed fire have become more favorable
in Texas as a whole (Kreuter et al. 2008) and in areas affected by the EAC (D. Lucia,
Texas Parks and Wildlife, personal communication). The combination of varying grazing
regimes and periodic prescribed fire in the Texas panhandle would provide a mosaic of
grassland patches in varying stages of recovery from disturbance, and offer a wide variety
of niches for grassland birds.
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32 Texas Tech University, Anthony Roberts, May 2009
Table 2.1. Avian community measures for breeding season birds among burned and unburned plots associated with the East Amarillo
Complex wildfires of 2006.
Shortgrass Burned (n=5) Mixedgrass Unburned (n=5) Species Abundances (#/ha) Burned (n=5) Unburned (n=5) Species Abundances (#/ha) 2007 1.806 1.425 2007 1.164 1.702 2008 1.714 1.220 2008 1.570 1.446 Species Richness Species Richness 2007 11 12 2007 9 11 2008 14 10 2008 13 13 Shannon Diversity Index (H’) Shannon Diversity Index (H’) 2007 1.165 ± 0.119 1.336 ± 0.092 2007 1.470 ± 0.075 1.442 ± 0.067 2008 1.669 ± 0.071 1.582 ± 0.070 2008 1.605 ± 0.078 1.804 ± 0.061 Simpson Diversity Index (D) Simpson Diversity Index (D) 2007 5.455 4.387 2007 5.306 5.528 2008 6.834 5.629 2008 5.662 7.619 Species Evenness (E) Species Evenness (E) 2007 0.589 0.693 2007 0.826 0.782 2008 0.798 0.835 2008 0.753 0.853 33 Texas Tech University, Anthony Roberts, May 2009
Table 2.2. Avian abundance and percent composition (in parenthesis) on burned and unburned shortgrass plots associated with the
East Amarillo Complex wildfires of 2006.
Shortgrass
2007 2008 Total Species Burned (n=5) Unburned (n=5) Burned (n=5) Unburned (n=5) Barn Swallow 1 (0.7) 1 (0.9) 2 (1.5) 0 4 (0.8) Brown‐headed Cowbird 2 (1.4) 1 (0.9) 6 (4.6) 5 (4.3) 14 (2.9) Cassin’s sparrow 4 (2.9) 2 (1.9) 6 (4.6) 6 (5.2) 18 (3.7) Cliff Swallow 0 1 (0.9) 1 (0.8) 0 2 (0.4) Common Grackle 0 0 2 (1.5) 0 2 (0.4) Common Nighthawk 8 (5.8) 1 (0.9) 7 (5.3) 5 (4.3) 21 (4.3) Dickcissel 2 (1.4) 1 (0.9) 0 0 3 (0.6) Grasshopper Sparrow 33 (23.9) 33 (31.7) 27 (20.6) 30 (25.9) 124 (25.3) Horned Lark 31 (22.4) 15 (14.4) 25 (19.1) 15 (12.9) 86 (17.6) Killdeer 1 (0.7) 0 1 (0.8) 1 (0.8) 3 (0.6) Lark Sparrow 26 (18.8) 11 (10.6) 21 (16.0) 12 (10.3) 70 (14.3) Mourning Dove 0 0 4 (3.1) 5 (4.3) 9 (1.8) Northern Bobwhite 0 0 1 (0.8) 0 1 (0.2) Scaled Quail 0 2 (1.9) 0 0 2 (0.4) Scissor‐tailed Flycatcher 2 (1.4) 3 (2.9) 2 (1.5) 4 (3.4) 11 (2.2) Western Meadowlark 28 (20.3) 33 (31.7) 26 (19.8) 33 (28.4) 120 (24.5) Total 138 131 104 89 490 34 Texas Tech University, Anthony Roberts, May 2009
Table 2.3. Matrix of similarity using Jaccard’s index (Cn) between years and burn conditions for
both shortgrass and mixedgrass sites associated with the East Amarillo Complex wildfires of
2006.
Shortgrass 2007 2007 2008 2008 Burned Unburned Burned Unburned Burned 1.000 0.769 0.563 ― Unburned 0.769 1.000 ― 0.571 Burned 0.563 ― 1.000 0.714 Unburned ― 0.571 0.714 1.000 Mixedgrass 2007 2007 2008 2008 Burned Unburned Burned Unburned Burned 1.000 0.667 0.571 ― Unburned 0.667 1.000 ― 0.500 Burned 0.571 ― 1.000 0.444 Unburned ― 0.500 0.444 1.000 Table 2.4. Densities (number per hectare ± SE) for the three most common species detected
during the breeding season on burned and unburned shortgrass plots associated with the East
Amarillo Complex wildfires of 2006.
Shortgrass Grasshopper Sparrow 2007 2008 Horned Lark 2007 2008 Western Meadowlark 2007 2008 Burned Unburned 0.487 (0.014) 0.384 (0.012) 0.562 (0.018) 0.387 (0.013) 0.325 (0.017) 0.325 (0.036) 0.406 (0.061) 0.144 (0.016) 0.360 (0.009) 0.257 (0.012) 0.579 (0.013) 0.377 (0.010) 35 Texas Tech University, Anthony Roberts, May 2009
Table 2.5. Winter avian abundance and percent composition (in parenthesis) on shortgrass plots
associated with the East Amarillo Complex wildfires of 2006.
Shortgrass Burned Unburned Total 2007 2008 2007 2008 Horned Lark 0 15 (68.2) 0 3 (21.4) 18 (50.0) McCown’s Longspur 0 1 (4.5) 0 1 (7.1) 2 (5.6) Scaled Quail 0 0 0 2 (14.3) 2 (5.6) Unknown Longspur 0 3 (13.6) 0 0 3 (8.3) Unknown Sparrow 0 2 (9.1) 0 0 2 (5.6) Western Meadowlark 0 1 (4.5) 0 8 (57.1) 9 (25.0) Total 0 22 0 14 36 36 Texas Tech University, Anthony Roberts, May 2009
Table 2.6. Avian abundance and percent composition (in parenthesis) on burned and unburned mixedgrass plots associated with the
East Amarillo Complex wildfires of 2006.
Mixedgrass 2007
2008
Total
Species Burned (n=5) Unburned (n=5) Burned (n=5) Unburned (n=5) Barn Swallow 0 5 (3.8) 1 (0.8) 0 6 (1.2) Brown‐headed Cowbird 4 (4.5) 1 (0.8) 2 (1.7) 8 (5.7) 15 (3.1) Blue Grosbeak 0 1 (0.8) 0 0 1 (0.2) Bullock’s Oriole 0 0 0 2 (1.4) 2 (0.4) Cassin’s Sparrow 22 (24.7) 28 (21.1) 31 (25.8) 27 (19.3) 108 (22.4) Cliff Swallow 0 0 0 3 (2.1) 3 (0.6) Common Nighthawk 4 (4.5) 7 (5.3) 12 (10.0) 7 (5.0) 30 (6.2) Dickcissel 3 (3.4) 3 (2.3) 0 0 6 (1.2) Eastern Meadowlark 0 0 0 3 (2.1) 3 (0.6) Eastern Kingbird 0 0 1 (0.8) 0 1 (0.2) Grasshopper Sparrow 9 (10.1) 35 (26.3) 23 (19.2) 22 (15.7) 89 (18.5) Horned Lark 0 0 0 1 (0.7) 1 (0.2) Killdeer 0 0 2 (1.7) 0 2 (0.4) Lark Sparrow 18 (20.2) 19 (14.3) 12 (10.0) 12 (8.6) 61 (12.7) Lesser Prairie‐chicken 0 0 1 (0.8) 0 1 (0.2) Mourning Dove 0 3 (2.3) 2 (1.7) 16 (11.4) 21 (4.4) Northern Bobwhite 1 (1.1) 2 (1.5) 2 (1.7) 6 (4.3) 11 (2.3) Scissor‐tailed Flycatcher 3 (3.4) 0 2 (1.7) 3 (2.1) 8 (1.7) Western Meadowlark 25 (28.1) 29 (21.8) 29 (25.2) 30 (21.4) 113 (23.4) Total 89 120 133 140 482 37 Texas Tech University, Anthony Roberts, May 2009
Table 2.7. Densities (number per hectare ± SE) for the three most common species
detected during the breeding season on burned and unburned mixedgrass plots associated
with the East Amarillo Complex wildfires of 2006.
Mixedgrass Grasshopper Sparrow 2007 2008 Cassin's Sparrow 2007 2008 Western Meadowlark 2007 2008 Burned Unburned 0.129 (0.013) 0.488 (0.012) 0.473 (0.020) 0.382 (0.018) 0.277 (0.011) 0.404 (0.013) 0.332 (0.013) 0.332 (0.013) 0.302 (0.010) 0.404 (0.013) 0.322 (0.011) 0.339 (0.010) Table 2.8. Winter avian abundance and percent composition (in parenthesis) on
mixedgrass plots associated with the East Amarillo Complex wildfires of 2006.
Mixedgrass Burned Unburned Total 2007 2008 2007 2008 Chestnut‐collared Longspur 0 0 0 3 (23.1) 3 (7.1) Horned Lark 0 0 1 (20.0) 0 1 (2.4) McCown’s Longspur 0 6 (100) 0 0 6 (14.3) Mourning Dove 1 (5.6) 0 0 0 1 (2.4) Unknown Longspur 15 (83.3) 0 4 (80.0) 1 (7.7) 20 (47.6) Unknown Sparrow 0 0 0 3 (23.1) 3 (7.1) Vesper Sparrow 1 (5.6) 0 0 0 1 (2.4) Western Meadowlark 1 (5.6) 0 0 6 (46.2) 7 (16.7) Total 18 6 5 13 42 38 Texas Tech University, Anthony Roberts, May 2009
CHAPTER III
RESPONSE OF NESTING GRASSLAND BIRDS TO LARGE-SCALE WILDFIRE
IN THE TEXAS PANHANDLE
ABSTRACT
Two large-scale wildfires occurred in the spring of 2006 in the Texas Panhandle
and burned over 360,000 ha of predominantly private ranchland. These fires are now
known as the East Amarillo Complex (EAC) wildfires. Contrary to the number of studies
of prescribed fire on the Great Plains, there have been few studies of the affects of
wildfire on prairie communities, including declining populations of grassland birds. I
studied the impact of these wildfires on nesting grassland birds on shortgrass and
mixedgrass prairie sites during the second and third years following the fires. On
shortgrass sites, overall passerine nest success was higher on unburned areas in 2007, but
by 2008, three breeding seasons after the fires, nest success was similar between burn
conditions. On mixedgrass plots Cassin’s sparrows (Aimophila cassinii) and grasshopper
sparrows (Ammodramus savannarum) were more successful on burned plots in 2007,
though had similar nest success on burned and unburned plots in 2008. Nest site
placement for the most common nesting species was similar between burn conditions
both years, however, vegetation around nests varied considerably from random points.
The composition of breeding birds, as measured by presence of nests, seemed to be
similar regardless of burn history on both shortgrass and mixedgrass plots. Though the
39 Texas Tech University, Anthony Roberts, May 2009
EAC wildfires affected people living in this area tremendously, they had few if any shortterm negative impacts on the avian community. In fact, they were more than likely
beneficial in providing similar services as historic fire regimes on the Southern High
Plains. Grassland birds have evolved life-history strategies to inhabit a landscape shaped
by frequent disturbances such as wildfire.
INTRODUCTION
On 12 March 2006 two large-scale wildfires were ignited east of Amarillo
in the Texas panhandle. Higher than normal rainfall in the spring of 2005 resulting in
abundant herbaceous growth followed by drought conditions during the subsequent
summer and fall produced high fine fuel loads. This resulted in ideal conditions for a
large wildfire (Van Speybroeck et al. 2006). The two fires started approximately ten
minutes apart, around 11 am, and are known as the Borger and I-40 fires. The Borger fire
occurred primarily in Hutchinson and Roberts counties and the I-40 fire burned in Gray
and Wheeler counties. Together they burned over 360,000 ha of predominantly private
ranch lands in what is known as the East Amarillo Complex (EAC) (Zane et al. 2006).
Losses from these fires included twelve lives, 22 homes and over 2,500 head of livestock
(Zane et al. 2006). Both fires were contained four days after ignition, leaving more than
360,000 ha of bare earth during the historically windy spring season on the Southern
High Plains (Van Speybroeck et al. 2006). Widespread vegetation loss on the mixedgrass
and shortgrass ecosystems had potentially large negative impacts on many species
including lesser prairie-chicken (Tympanuchus pallidicinctus), mountain plover
40 Texas Tech University, Anthony Roberts, May 2009
(Chanadrius morinellus), and numerous species of grassland obligate songbirds that
breed in the Southern High Plains.
There have been numerous studies illustrating the decline of grassland birds over
the last sixty years, the most widely cited being Knopf (1994), who stated that this guild
has experienced the most consistent and widespread declines of any guild of birds in
North America. Among these are numerous high priority species in the Southern High
Plains such as mountain plover, long-billed curlew (Numenius americanus), Cassin’s
sparrow (Aimophila cassinii), and lark sparrow (Chondestes grammacus) ( Johnsgard
1973, Knopf 1994). In addition, birds of the shortgrass prairies are some of the least
ecologically understood of all grassland birds (Askins et al. 2007). Multiple
anthropogenic forces have contributed to long-term declines in grassland bird
populations, especially habitat loss and fragmentation due to urban development,
agriculture, and infrastructure (Herkert 1995, Peterjohn and Sauer 1999). In the Texas
panhandle, where the EAC wildfires occurred, these anthropogenic forces have occurred
to a lesser degree than in other parts of the Great Plains. Contrary to fragmented areas of
the Great Plains, the problem in the Texas Panhandle may not be strictly linked to habitat
loss, but instead could be linked to fire suppression. Fire may be an important tool in
restoring the native vegetation and microclimate characteristics (Brockway et al. 2002),
and ultimately increasing avian productivity in grasslands. This is especially true in the
Texas Panhandle where native rangeland is a dominant part of the landscape and could
potentially be a stronghold for grassland birds on the Southern High Plains.
Previous studies of grassland bird response to fire have focused on the effects of
prescribed fire (e.g. Huber and Steuter 1984, Pylypec 1991, Madden et al. 1999,
41 Texas Tech University, Anthony Roberts, May 2009
Rohrbaugh et al. 1999, Vickery et al. 1999). Relatively few studies have assessed the
avian response following major wildfires in grasslands. Wildfires may be more intense
and cover larger spatial scales than prescribed fires, and therefore have a unique effect on
the avian community (Smith 2000). A grassland wildfire study in Arizona found a
decrease in litter cover drove the largest individual avian species responses in burned
areas, though this study focused more on the suitability of introduced grasses after fire
than strictly population changes on native grasslands (Bock and Bock’s 1992).
The EAC presented a unique opportunity to study the effects of large-scale
wildfires on grassland bird ecology. Much of the work on grassland birds and fire has
focused on tallgrass prairies and northern mixedgrass prairies, where climate differs from
the Texas panhandle. Comparatively little is known about how nesting birds on the
Southern High Plains react to fire, whether wild or prescribed. It is thought that fire was
historically the principle ecological driver on tallgrass prairies. This contrasts with
shortgrass prairies where drought and grazing were probably more important than fire
(Milchunas et al. 1988), making the effects of fire in this area much different than those
seen in tallgrass ecosystems (Askins et al. 2007). The continuing cycle of bison (Bos
bison) grazing and fire described by Fuhlendorf and Engel (2001) may have greatly
affected ground nesting birds historically and the EAC gives us an opportunity to
examine a piece of that relationship. To better understand the benefits of fire on this
landscape I studied grassland bird nesting ecology on areas affected by the EAC during
the second and third years after the fires. I searched for and monitored nests in shortgrass
and mixedgrass prairie sites to provide a thorough account of nesting ecology in the
Southern High Plains. There were no studies being conducted in this area prior to March
42 Texas Tech University, Anthony Roberts, May 2009
of 2006 so I compared populations on burned sites with populations on proximate,
similar, unburned sites.
STUDY AREA
I surveyed grassland birds on private ranches in Roberts, Gray, and Donley
counties in the Texas panhandle. These counties lay in a transition zone between
shortgrass and mixedgrass prairie types in the Southern High Plains in an area known as
the Llano Estacado. The Llano Estacado occurs in Texas and New Mexico and is
bordered by the Canadian River to the north and the Caprock Escarpment in the south
and east. The landscape is characterized by rolling hills leading to flat plains and is
interspersed with ephemeral wetland depressions known as playas (Williams and Welker
1966, Wyrick 1981). Elevation ranges from 300 m to over 900 m (Wyrick 1981). Along
the edge of the flat plains and rolling hills there are often steep bluffs traversed by
canyons and drainage channels that contain different vegetation communities due to
changes in aspect, water availability, and soil types (Williams and Welker 1966).
Climate in this area is characterized by hot summers and cold to mild winters
(Wyrick 1981). Mean summer temperature is 25 C and mean winter temperature is 2 C
but temperatures fluctuate extensively within seasons (Williams and Welker 1966). This
area receives on average 53 cm of precipitation a year (Williams and Welker 1966) and
rainfall typically occurs in the form of thunderstorms, and precipitation may be absent for
weeks or months during the summer, causing drought conditions (Borchert 1950).
Western shortgrass prairie receives markedly less precipitation than the eastern Great
43 Texas Tech University, Anthony Roberts, May 2009
Plains and over 80% of moisture occurs between April and September (Williams and
Crump 1980).
Pre-European settlement vegetation of this area was mainly blue grama
(Bouteloua gracilis) and sideoats grama (Bouteloua curtipendula) with low lands and
sandy soils containing more little bluestem (Schizachyrium scoparium), switchgrass
(Panicum virgatum), and indiangrass (Sorghastrum nutans), but agriculture has converted
a large portion of the native prairie into monoculture croplands (Wyrick 1981). Shrubs
include mesquite (Prosopis glandulosa) and red-berry juniper (Juniperis pinchotii) in
tight upland soils and sand-shinnery oak (Quercus havardii) and sand sagebrush
(Artemesia filifolia) in less dense, sandier soils. The EAC impacted two bird conservation
regions, the shortgrass prairie (BCR 18) and the central mixed-grass prairie (BCR 19)
(USFWS 2000).
METHODS
Study Plot Selection
Selection of the sixteen plots used in this study was based on access to private
lands, burn history, and vegetation community type, either shortgrass or mixedgrass
prairie. The latter distinction was made by examining soil type and dominant vegetation.
Burn history was established by talking with landowners and local officials and looking
for fire scars on any woody vegetation in the area such as prickly pear cactus (Opuntia
spp), catclaw mimosa (Mimosa aculeaticarpa), or sand sagebrush. The sixteen plots were
equally distributed among burned and unburned areas in shortgrass and mixedgrass
prairie. This resulted in a study design with four replicates within each of four treatments,
44 Texas Tech University, Anthony Roberts, May 2009
burned shortgrass, unburned shortgrass, burned mixedgrass, and unburned mixedgrass.
Individual plots were placed at least 1 km apart and 0.5 km from the known fire
boundary, roads, or other vegetation or topology changes. All work was done in 2007 and
2008, two and three breeding seasons after the fires respectively.
Nesting Success and Characteristics
I established a 4 ha (200 m × 200 m) nest search area within each of the sixteen
study plots. Within these areas I conducted focused searches for nests of ground-nesting
birds with the search-line method (Berthelsen et al. 1989). The area was traversed
systematically by one to four individuals walking slowly while parting the grass with
sticks to facilitate flushing nesting females and detecting nests (Berthelsen et al. 1989).
During this study five different people performed all nest searches. If individuals did not
have prior experience searching for nests in grasslands they were trained extensively
before being allowed to search independantly. Subsequent to systematic searches, I
walked the plot in a haphazard route to focus attention on behavioral cues of individual
birds. This involved closely watching individual adults and observing clues that may
indicate a nest is nearby (Martin and Geupel 1993, Winter et al. 2003). Behavioral clues
include intensely chipping adults, adults flushing close to the observer and flying a short
distance, or adults carrying nest material, food or fecal sacs (Winter 1999). Nest
searching was not done during precipitation to reduce the risks of missing nests, putting
nests at risk by flushing females during inclement weather, and damaging wet vegetation
(Winter 1999). I marked nest locations by placing a small piece of flagging ≥5m north
and south of the nest. Subsequent monitoring occurred every 3 to 4 days until a nest
45 Texas Tech University, Anthony Roberts, May 2009
outcome was established (Martin and Geupel 1993, Ralph et al. 1993). I considered a
nest successful if 1 nestling fledged.
Vegetation Sampling
I measured nest site vegetation within five days of nest fate to reduce biases due
to vegetation changes after fate date. I used methods established by Daubenmire (1959)
and Nudds (1977), respectively, to determine horizontal and vertical vegetation cover. I
measured percent cover of grass, forbs, litter, bare ground, and woody plants using a
modified Daubenmire frame (50 cm × 50 cm) at the nest and at 1 m in each cardinal
direction. To estimate vertical obstruction, I used a modified vegetation profile board
(Nudds 1977) that was read from 4 m in each cardinal direction at 1 m above the ground.
The profile board was 2 m tall by 10 cm wide and was separated into 20 dm sections. I
measured percent coverage at each dm of the profile board as well as recording the
highest point reached by living or dead vegetation. I repeated each vegetation
measurement at a random point within 25 m of the nest in a direction and distance
determined by use of a random number table. I also recorded the distance to the nearest
woody plant species from the nest and the random point. Nests that were built within a
woody plant species were recorded as 0.01 m to nearest woody vegetation.
Data Analysis
Nest success rates were calculated for incubation, nestling, and total nesting
periods using Mayfield’s (1975) estimates. Standard errors for nest success rates were
46 Texas Tech University, Anthony Roberts, May 2009
calculated following Johnson (1979). I calculated nest success for all passerines
combined as well as for individual species where sample sizes permitted. I obtained
duration of each nest stage for each species from Baicich and Harrison (2005). Small
sample sizes did not allow for individual plot estimates of nest success and hence
standard hypothesis tests were difficult to use. Instead I used standard errors to place a
confidence interval around the difference of the mean densities to test for differences
between burn condition and year (Zar 1999). This is similar to a t-test, so that when a ttest is significant, the confidence interval does not include zero, and when the t-test is not
significant the confidence interval includes zero. Confidence intervals presented here
illustrate the size of the difference between two treatments. I made no attempt to compare
differences between shortgrass and mixedgrass plots.
I analyzed nest site vegetation data using PROC MIXED in SAS (SAS Institute
2000). In all cases a probability level of 0.05 was used to determine significance. I
compared nests to random sites, nests on burned plots compared to nests on unburned
plots, and the combination of the two (i.e. nests on unburned plots compared to random
points on unburned plots). Analysis of Daubenmire frame data used an average of the
center and the four cardinal directions for each ground cover category and vertical
densities were analyzed at each cardinal direction and at each dm. Vegetation placement
around nests may be important in controlling the effects of the sun and wind (With and
Webb 1993) so it is important to examine the directional placement and density of
vegetation around nests.
RESULTS
47 Texas Tech University, Anthony Roberts, May 2009
Shortgrass
On shortgrass plots I found a total of 72 nests of nine different species throughout
two field seasons (Table 3.1). Grasshopper sparrow (Ammodramus savannarum), lark
sparrow, horned lark (Eremophila alpestris), and western meadowlark (Sturnella
neglecta) accounted for 89% of all nests found. Lark bunting (Calamospiza melanocorys)
and common nighthawk (Chordeiles minor) were each represented by a single nest in
2007, and none were found the following year (Table 3.1). I found a single nest of
Cassin’s sparrow and mourning dove (Zenaida macroura) in 2008, though none the
previous year. The number of nests found was relatively consistent between burn
condition and year.
Nest success.— Total passerine nest success was higher on unburned plots in 2007 (0.211
< 0.219 < 0.228) (Table 3.2). In contrast, nest success was similar across burn treatments
in 2008 but was significantly lower than in 2007 on both burned (0.076 < 0.094 < 0.104)
and unburned (0.316 < 0.317 < 0.338) plots. In all cases nest success during the nestling
stage was higher than during the incubation stage (Table 3.2). Brown-headed cowbird
(Molothrus ater) parasitism rates were low in both 2007 (18%) and 2008 (11%) and were
restricted to lark sparrows and horned larks. All horned lark nests were parasitized in
2007 (n = 3) compared to 33% of nine nests in 2008 (n = 3).
Due to small sample sizes I was only able to compare individual species nest
success rates using lark sparrow nests. In 2007 lark sparrow nest success was similar
across burn treatments, with 32.4% success on burned plots and 34.5% success on
unburned plots. During the 2008 breeding season, lark sparrow nest success was lower on
48 Texas Tech University, Anthony Roberts, May 2009
burned plots (14.9%) than on unburned plots (31.9%) (0.095 < 0.170 < 0.245). There was
no change in nest success between years on unburned plots, but nest success on burned
plots was lower in 2008 (0.129 < 0.175 < 0.221).
Nest site characteristics.— On shortgrass plots I was able to find enough nests to perform
nest site vegetation analysis for grasshopper sparrow and lark sparrow. I restricted
analysis of vertical cover to estimates 50 cm and below. In 2007 there were no
differences in grasshopper sparrow nests in any of the five ground cover measures of
grass, forbs, woody vegetation, litter, or bare ground between burned and unburned sites,
between nest and random points, or the combination of the two (Table 3.3). There was
also no difference in the distance to nearest woody vegetation (Table 3.3). All vertical
cover estimates were similar except from the east at 20 cm. At this height nests on burned
plots had lower coverage (14%) than nests on unburned plots (63%) (F1,12 = 5.07, P =
0.04). In 2008 only percent forbs was significantly higher at nests on burned plots
compared to nests on unburned plots (F1,4 = 23.38, P = 0.008). All vertical coverage
estimates were similar across burn conditions, between nests and random points, and the
combination of the two.
Lark sparrow nests showed no differences in 2007 between burn conditions
within the five ground cover variables or vertical densities measured (Table 3.4). Nests
were placed closer to woody vegetation than random points in 2007. Vertical cover
differed from random sites in multiple ways. Nest sites had higher percent cover at 20 cm
to the east (F1,13.8 = 39.48, P < 0.0001), south (F1,18 = 59.04, P < 0.0001), and west (F1,18 =
29.87, P < 0.0001) and at 30 cm to the east (F1,18 = 8.41, P = 0.009) and the west (F1,18 =
49 Texas Tech University, Anthony Roberts, May 2009
7.19, P = 0.015). Woody vegetation coverage in 2008 was higher around nests on
unburned plots than nests on burned plots (F1,10 = 62.62, P < 0.0001). All other ground
coverage estimates were similar between and within burn conditions (Table 3.4). There
was tremendous variation in vertical coverage estimates within all combinations. Nests
had higher percent cover than random sites at 20 cm from the north (F1,10 = 9.41, P =
0.012), east (F1,9 = 5.95, P = 0.037), and west (F1,9 = 44.38, P < 0.0001), at 30 cm from
the north (F1,10 = 7.97, P = 0.018) and west (F1,10 = 48.24, P < 0.0001), at 40 cm from the
north (F1,10 = 5.39, P = 0.043) and west (F1,10 = 31.63, P = 0.0002), and at 50 cm from the
north ((F1,10 = 8, P = 0.018). Nests on unburned plots had higher percent cover than those
on burned plots from the east at 30 cm (F1,10 = 9.4, P = 0.012), 40 cm (F1,10 = 9.45, P =
0.012), and at 50 cm (F1,10 = 10.41, P = 0.009), from the north at 50 cm (F1,10 = 6.31, P =
0.031), and from the west at 30 cm (F1,10 = 12.06, P = 0.006). Nests on unburned sites had
higher percent cover than their associated random points from the north at 50 cm (F1,10 =
6.96, P = 0.025) and from the west at 20 cm (F1,9 = 6.57, P = 0.030), 30 cm (F1,10 = 14.35,
P = 0.003), and 40 cm (F1,10 = 19.04, P = 0.001).
Mixedgrass
On mixedgrass plots I found a total of 83 nests of seven different species (Table
3.5). Cassin’s sparrow, grasshopper sparrow, western meadowlark and mourning dove
represented 84% of all nests found over the two field seasons. One scissor-tailed
flycatcher (Tyrannus forficatus) nest was found in 2007 and none in 2008. The number of
nests found at each burn condition and year was not significantly different, except for
50 Texas Tech University, Anthony Roberts, May 2009
more nests (n = 42) were found in burned areas in 2008 than in any other condition or
year (Table 3.5).
Nest success.— Passerine nest success in 2007 was higher on burned areas than unburned
areas (0.109 < 0.121 < 0.133) (Table 3.2). In 2008 the opposite was true as unburned
areas had higher nest success (0.053 < 0.062 < 0.069), but markedly fewer nests than
burned areas (Table 3.2). In most cases, success rates during the nestling stage were
higher than during the incubation stage (Table 3.2). Parasitism by brown-headed
cowbirds was restricted to Cassin’s sparrows with 43% parasitism rates in 2007 and 11%
in 2008 (9% and 11% overall passerine parasitism rate).
On mixedgrass sites I only had a large enough sample to evaluate nest success
over both conditions and years for Cassin’s sparrow. In 2007 nests on burned sites had a
higher success rate (52.9%) than nests on unburned sites (20.2%) (0.231 < 0.328 <
0.426). In 2008, however, there was no difference between burn plots (32.4%) and
unburned plots (34.6%), but nest success increased from 2007 on unburned plots (0.069 <
0.144 < 0.219) and decreased from 2007 on burned plots (0.166 < 0.206 < 0.246).
Grasshopper sparrows followed the same pattern as Cassin’s sparrows in 2007,
with nesting success higher on burned plots (61.0%) than unburned plots (22.7%) (0.288
< 0.383 < 0.479). Nesting success was higher on burned plots in 2007 compared to
burned plots in 2008 (36.1%) (0.184 < 0.251 < 0.311). No nests of grasshopper sparrows
were found on unburned sites in 2008, so I was unable to compare burn treatments in that
year.
51 Texas Tech University, Anthony Roberts, May 2009
Nest site characteristics.— I analyzed nest site vegetation characteristics for three species
on mixedgrass plots; grasshopper sparrow, Cassin’s sparrow, and lark sparrow. For
grasshopper sparrows in 2007, litter cover was higher on nests on unburned plots than at
nests on burned plots (F1,12 = 6.51, P = 0.020) (Table 3.6). There were no differences in
the other four horizontal coverage variables or distance to woody vegetation within or
between burn conditions. There were also no differences in vertical vegetation coverage
at any height. In 2008 no grasshopper sparrow nests were found on unburned plots so
only comparisons between nests and random points on burned plots were possible. None
of the five ground coverage estimates were significantly different in 2008. Vertical
coverage was higher at nests than at random points at 20 cm from the east (F1,16.6 = 4.82,
P = 0.043), south (F1,16.5 = 18.2, P = 0.001), and west (F1,16.7 = 7.08, P = 0.017), and at 30
cm from the south (F1,16.5 = 7.7, P = 0.013).
Cassin’s sparrow nests showed no differences in the five coverage variables
measured between burn conditions, nests and random points, or the combination of the
two in 2007 (Table 3.7). Nests of Cassin’s sparrows were placed closer to woody
vegetation than random points. Nests had higher vertical coverage estimates than random
points from the north at 40 cm (F1,9 = 6.04, P = 0.036) and at 50 cm (F1,10 = 8.18, P =
0.017), from the east at 20 cm (F1,10 = 6.6, P = 0.028) and at 40 cm (F1,10 = 23.73, P =
0.001), from the south at 20 cm (F1,10 = 8.47, P = 0.015), 30 cm (F1,10 = 21.54, P = 0.001),
and 40 cm (F1,10 = 18.74, P = 0.002), and from the west at 40 cm (F1,10 = 16.64, P =
0.002). Again in 2008 there were no differences between horizontal coverage for Cassin’s
sparrow nests (Table 3.7). There was also no difference in distance to woody vegetation.
Vertical coverage estimates varied even more in 2008 than in 2007. Vertical cover was
52 Texas Tech University, Anthony Roberts, May 2009
denser in all directions at nearly all heights, excluding 10 cm and 50 cm. Nests had higher
cover estimates from the north at 20 cm (F1,21.6 = 13.85, P = 0.001), 30 cm (F1,24 = 18.87,
P = 0.0002), and 40 cm (F1,22 = 9.97, P = 0.005), from the east at 20 cm (F1,24 = 4.54, P =
0.043) and 30 cm (F1,24 = 4.49, P = 0.044), from the south at 20 cm (F1,3.27 = 11, P =
0.039), 30 cm (F1,6.72 = 13.1, P = 0.009), and 40 cm (F1,9.3 = 9.47, P = 0.012), and from the
west at 20 cm (F1,24 = 17.66, P = 0.0003), 30 cm (F1,24 = 15.69, P = 0.001), and 40 cm
(F1,10.3 = 9.05, P = 0.013).
Only one horizontal coverage difference was measured between lark sparrow
nests in 2007. Lark sparrow nests built on unburned areas had higher litter cover than
those on burned areas (Table 3.8). All other horizontal coverage measurements and
distance to woody vegetation were similar across burn treatments and between nests and
random points. Vertical cover was denser at nests than at random points from the west at
20 cm (F1,8 = 5.86, P = 0.042). All other vertical densities were similar in 2007. In 2008
there were no differences in the five ground cover estimates. Vertical density was higher
at nests compared to random points from the north at 10 cm (F1,12 = 16.32, P = 0.002) and
20 cm (F1,8.49 = 24.38, P = 0.001), from the east at 10 cm (F1,12 = 53.96, P < 0.0001) and
20 cm (F1,12 = 15.34, P = 0.002), from the south at 10 cm (F1,12 = 18.98, P = 0.001) and 20
cm (F1,12 = 25.76, P = 0.0003), and from the west at 10 cm (F1,8.76 = 34.15, P = 0.0003)
and 20 cm (F1,18 = 7.19, P = 0.015). Nests on burned plots had lower vertical cover than
those built on unburned plots from the east at 10 cm (F1,12 = 29.82, P = 0.0001). Within
burn plots in 2008 nest had higher vertical cover than random points from the east at 10
cm (F1,12 = 26.3, P = 0.0002) and 20 cm (F1,12 = 6.44, P = 0.026).
53 Texas Tech University, Anthony Roberts, May 2009
DISCUSSION
Across the study area there was above average rainfall in 2007, a year after the
fires. Though the Texas panhandle receives on average 53 cm of precipitation a year, in
2007 75 cm of precipitation fell, including 38 cm before breeding season started in May
(West Texas Mesonet 2007). This probably contributed to increased plant growth and
affected nest success and nest site selection across the study area. The composition of
breeding birds, as measured by presence of nests, seemed to be similar regardless of burn
history on both shortgrass and mixedgrass plots. Nesting success across the landscape
was similar to those reported in other studies in the Texas panhandle (Berthelson and
Smith 1995, Thompson 2003), but higher than other grassland bird studies (e.g.,
Rohrbaugh et al. 1999, Fondell and Ball 2004). Many different variables could explain
high nest success though it is probably partly due to the large amounts of undisturbed
rangeland in the Texas panhandle that have fewer edges and hence fewer nest predators
such as the Virginia opossum (Didelphis virginiana), striped skunk (Mephitis mephitis),
or red fox (Vulpes vulpes).
Differences of nest placement on shortgrass plots between burned and unburned
sites were few. On shortgrass plots, grasshopper sparrows nested in areas with denser
cover east of the nest in 2007 on unburned plots than on burned plots. More differences
were noted in 2008. For example, forb cover around grasshopper sparrow nests was
higher at nests in burned plots. Lark sparrow nests had higher woody cover at nests on
unburned plots. They also seemed to nest in much denser vegetation on unburned plots,
with vegetation concentrated on the east and west sides of a nest. This is consistent with
54 Texas Tech University, Anthony Roberts, May 2009
habitat preferences shown in other parts of their range. In Montana, lark sparrows
avoided areas with little or no woody vegetation after a wildfire (Bock and Bock 1987).
Overall there were more differences between nest sites and random sites than
between burn conditions. This suggests that individual species were selecting a specific
site within the landscape, and that site type was available on both burned and unburned
areas two breeding seasons after the EAC wildfires. Individual measurements of ground
cover showed little variation between nest sites and random sites on shortgrass plots.
Vertical cover seemed to be more important for all bird species, as nests were placed in
taller, denser vegetation such as little bluestem or three awn (Aristida spp), rather than the
dominant blue grama and buffalo grass (Bouteloua dactyloides). Lark sparrows showed
the greatest preference for dense vertical vegetation. Nest sites had denser vegetation than
random sites in 2007 from the east, south and west. In 2008 nest sites had denser vertical
vegetation from the north as well. Dense vegetation may provide protection from extreme
heat or wind, and regulate the microclimate of the nest, especially in the extreme
conditions found on shortgrass prairies. Lusk et al.’s (2003) determination of lark
sparrow nest site selection showed this species prefers dense vegetation close to woody
plants for nesting.
On mixedgrass plots, similar to shortgrass plots, there were few measured
differences among nests on burned versus unburned plots. In 2007 litter cover around lark
sparrow nests on burned sites was lower than that on unburned sites. In 2008,
grasshopper sparrows had a higher amount of forb cover around nests at burned sites,
similar to this species’ nests on shortgrass plots. Vertical cover was denser on the east
side of lark sparrow nests built on unburned plots, relative to burned plots.
55 Texas Tech University, Anthony Roberts, May 2009
Again, there were many more differences between nest sites and random sites
than between or within burn conditions. Cassin’s sparrow’s nest sites had much denser
vegetation both years compared to random sites, particularly from the south and east in
2007. All vertical density differences held up in 2008 in addition to denser vegetation
from the west at nest sites. Dense vegetation could help shield nests from intense summer
sun, especially from the south and east. Cassin’s sparrows also nested in or close to
woody vegetation more than other species. This follows descriptions of nest sites of this
species across their range (Dunning et al. 1999). In 2008 grasshopper sparrow nests had
higher densities low to the ground from the east, south, and west. Lark sparrows
continued the trend and had denser vegetation all around the nest 20 cm and below.
As early as two breeding seasons after the wildfires, it seems as though there has
been little effect on the success of nests or any alteration of nest placement strategies by
individual species, possibly as a result of increased vegetation growth due to higher than
average rainfall. This would follow a similar timeline presented by Launchbaugh (1964)
for plants. His paper stated that vegetation recovery from dry season fire typically
requires no longer than three years. Though the EAC wildfires had tremendous affects on
the people living in this area, in this time frame they had few if any negative impacts on
the avian community. In fact, they were more than likely beneficial in providing similar
services as historic fire regimes on the Southern High Plains. Grassland birds have
evolved life-history strategies to survive and thrive on a landscape shaped by frequent
disturbances such as wildfire.
MANAGEMENT IMPLICATIONS
56 Texas Tech University, Anthony Roberts, May 2009
The success of grassland bird nests depends on a myriad of conditions, including
availability of food and nest sites, predator abundance, and environmental conditions.
How these various factors change in relation to large-scale wildfires has yet to be fully
realized. In this study I found that nest site selection varies little between different sites,
suggesting that if managers offer suitable conditions for nesting, these species will be
able to utilize the area. Rohrbaugh et al. (1999) came to a similar conclusion that burning
does not negatively affect reproductive success of many grassland birds. A mosaic of
patches in various successional stages offered by periodic burning would benefit the
entire avian community by offering a multitude of potential nesting sites.
Landscape heterogeneity is obtained by varying management regimes across
multiple property lines. Drought and grazing are already present on this landscape but fire
has yet to return to historic levels. Fortunately, landowner’s attitudes towards prescribed
fire have become more favorable in Texas as a whole (Kreuter et al. 2008) and in areas
affected by the EAC (D. Lucia, Texas Parks and Wildlife, personal communication).
Wildfire is potentially harmful to human lives and property and therefore is largely
unsuitable as a management option. Instead, prescribed fire has been used to mimic
wildfire effects and reduce wildfire potential (Pattison 1998). The effectiveness of fire
surrogates such as mowing in mimicking fire effects has not been fully researched on the
Great Plains. Though mowing would provide similar vegetation structure as fire, it would
probably fail to reduce litter cover or return nutrients to the soil and temporarily increase
vegetation growth resulting in increased nesting sites. The combination of varying
grazing regimes and periodic prescribed fire in the Texas panhandle would provide a
mosaic of grassland patches in varying stages of recovery from disturbance. This has
57 Texas Tech University, Anthony Roberts, May 2009
been shown here and suggested in previous studies of grassland nesting birds as well
(Herkert 1994; Rohrbaugh et al. 1999).
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61 Texas Tech University, Anthony Roberts, May 2009
Table 3.1. Species nesting abundance and percent composition (in parenthesis) on shortgrass plots associated with the East Amarillo
Complex wildfires of 2006.
Shortgrass 2007 2008 Total Burned (n=5) Unburned (n=5) Burned (n=5) Unburned (n=5) Cassin’s Sparrow 0 0 1 (5.6) 0 1 (1.4) Common Nighthawk 0 1 (4.2) 0 0 1 (1.4) Grasshopper Sparrow 2 (14.3) 8 (33.3) 2 (11.1) 4 (25.0) 16 (22.2) Horned Lark 3 (21.4) 0 6 (33.3) 3 (18.8) 12 (16.7) Lark Bunting 1 (7.4) 0 0 0 1 (1.4) Lark Sparrow 7 (50.0) 11 (45.8) 5 (27.8) 4 (25.0) 27 (37.5) Mourning Dove 0 0 1 (5.6) 0 1 (1.4) Scissor‐tailed Flycatcher 0 2 (8.3) 0 2 (12.5) 4 (5.6) Western Meadowlark 1 (7.4) 2 (8.3) 3 (16.7) 3 (18.8) 9 (12.5) Total 14 24 18 16 72 Number of Species 5 5 6 5 9 62 Texas Tech University, Anthony Roberts, May 2009
Table 3.2. Total passerine nest success using Mayfield’s estimate for both shortgrass and
mixedgrass plots associated with the East Amarillo Complex wildfires of 2006.
Shortgrass Burned (n=5) N Stage 2007 2008 Mayfield Success Unburned (n=5) N (SE) Mayfield Success (SE) Incubation 14 0.585 (0.02) 18 0.654 (0.02) Nestling 7 0.649 (0.03) 19 0.826 (0.01) Total 14 0.335 (0.02) 24 0.554 (0.01) Incubation 13 0.497 (0.03) 13 0.389 (0.03) Nestling 12 0.505 (0.03) 10 0.592 (0.03) Total 18 0.244 (0.02) 16 0.227 (0.02) Mixedgrass Burned (n=5) N Stage 2007 2008 Mayfield Success Unburned (n=5) N (SE) Mayfield Success (SE) Incubation 14 0.543 (0.02) 12 0.569 (0.02) Nestling 12 0.718 (0.02) 9 0.551 (0.03) Total 17 0.431 (0.01) 14 0.309 (0.02) Incubation 36 0.559 (0.01) 11 0.505 (0.03) Nestling 25 0.645 (0.02) 8 0.855 (0.02) Total 42 0.358 (0.01) 12 0.419 (0.02) 63 Texas Tech University, Anthony Roberts, May 2009
Table 3.3. Grasshopper sparrow nest site characteristics on burned and unburned shortgrass prairie sites associated with the East
Amarillo Complex wildfires of 2006. Means are for nests on burned (n = 4) and unburned (n = 12) plots as well as all nests (n = 16)
and all random points (n = 16).
Shortgrass Grass 2007 2008 Forbs Woody Litter Bareground Nearest Woody Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Burned 48.00 10.66 18.75 3.20 0 0 12.25 4.19 24.00 7.31 1.83 0.33 Unburned 65.12 7.89 16.13 1.85 0 0 9.32 3.07 11.71 5.76 1.22 0.62 Nest 57.89 7.73 16.75 2.62 0 0 11.55 3.08 15.46 4.97 1.40 0.45 Random 55.23 7.73 18.13 2.62 0 0 10.02 3.08 20.24 4.97 2.49 0.62 Burned 21.25 5.15 13.25 1.32 2.75 1.25 21.75 3.96 44.75 10.14 1.30 0.10 Unburned 41.75 3.72 4.20 1.32 1.25 1.25 30.75 3.96 20.25 7.25 1.46 1.45 Nest 32.00 4.49 7.75 1.32 1.25 1.25 26.25 3.96 32.75 7.67 1.38 0.59 Random 31.00 4.49 9.70 1.32 2.75 1.25 26.25 3.96 32.25 7.67 2.60 1.41 64 Texas Tech University, Anthony Roberts, May 2009
Table 3.4. Lark sparrow nest site characteristics on burned and unburned shortgrass prairie sites associated with the East Amarillo
Complex wildfires of 2006. Means are for nests on burned (n = 12) and unburned (n = 13) plots as well as all nests (n = 25) and all
random points (n = 25).
Shortgrass Grass 2007 2008 Forbs Woody Litter Bareground Nearest Woody
Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Burned 27.23 9.31 12.58 1.84 4.00 1.37 8.56 2.50 52.29 7.29 0.01 0 Unburned 31.42 16.97 10.78 2.03 4.75 1.86 12.92 3.38 13.17 10.36 0.18 0.17 Nest 29.62 10.07 13.85 1.94 6.29 1.63 11.04 2.51 43.42 7.39 0.12 0.09 Random 29.04 10.07 9.51 1.94 2.46 1.63 10.43 2.51 52.05 7.39 2.63 0.45 Burned 29.25 2.98 5.40 1.29 0.55 0.71 30.50 3.28 34.25 6.92 2.70 2.10 Unburned 30.00 3.44 5.53 1.59 9.17 0.82 23.50 3.78 33.5 7.99 0.01 0 Nest 30.00 3.22 6.10 1.34 4.63 0.77 26.75 3.54 33.87 7.48 1.55 1.25 Random 29.25 3.22 4.80 1.34 5.08 0.77 27.25 3.54 33.87 7.48 2.20 0.68 65 Texas Tech University, Anthony Roberts, May 2009
Table 3.5. Species nesting abundance and percent composition (in parenthesis) on mixedgrass plots associated with the East Amarillo
Complex wildfires of 2006.
Mixedgrass 2007 2008 Total Burned (n=5) Unburned (n=5) Burned (n=5) Unburned (n=5) Cassin’s Sparrow 3 (17.6) 4 (28.6) 16 (38.1) 5 (41.7) 28 (32.9) Common Nighthawk 1 (5.9) 1 (7.1) 1 (2.4) 1 (8.3) 4 (4.7) Grasshopper Sparrow 4 (23.5) 4 (28.6) 11 (26.2) 0 19 (22.4) Lark Sparrow 5 (29.4) 1 (7.1) 5 (11.9) 3 (25.0) 14 (16.5) Mourning Dove 2 (11.8) 4 (28.6) 4 (9.5) 1 (8.3) 11 (12.9) Scissor‐tailed Flycatcher 1 (5.9) 0 0 0 1 (1.2) Western Meadowlark 1 (5.9) 0 5 (11.9) 2 (16.7) 8 (9.4) Total 17 14 42 12 85 Number of Species 7 5 6 5 7 66 Texas Tech University, Anthony Roberts, May 2009
Table 3.6. Grasshopper sparrow nest site characteristics on burned and unburned mixedgrass prairie sites associated with the East
Amarillo Complex wildfires of 2006. Means are for nests on burned (n = 15) and unburned (n = 4) plots as well as all nests (n = 19)
and all random points (n = 19).
Mixedgrass Grass 2007 2008 Forbs Woody Litter Bareground Nearest Woody Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Burned 41.13 12.62 29.09 5.84 3.50 2.87 11.00 2.11 20.18 12.72 1.43 0.16 Unburned 35.63 15.35 10.04 6.97 7.37 2.87 18.62 2.11 30.92 15.49 1.58 0.27 Nest 40.47 10.08 18.48 5.26 4.87 2.87 16.00 2.11 24.59 10.21 1.52 0.15 Random 36.29 10.18 20.66 5.26 6.00 2.87 13.63 2.11 26.52 10.21 1.96 0.49 Burned 26.29 2.37 9.41 1.35 5.02 1.31 39.70 6.50 20.32 8.04 1.38 0.40 Unburned na na na na na na na na na na na na Nest 24.70 3.35 10.32 1.58 4.10 1.80 41.65 6.8 19.36 8.28 1.39 0.39 Random 27.87 3.35 8.49 1.58 5.94 1.85 37.75 6.80 21.26 8.28 1.25 0.25 67 Texas Tech University, Anthony Roberts, May 2009
Table 3.7. Cassin’s sparrow nest site characteristics on burned and unburned mixedgrass prairie sites associated with the East Amarillo
Complex wildfires of 2006. Means are for nests on burned (n = 18) and unburned (n = 8) plots as well as all nests (n = 26) and all
random points (n = 26).
Mixedgrass Grass 2007 2008 Forbs Woody Litter Bareground Nearest Woody Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Burned 24.33 2.97 15.83 10.57 10.00 10.45 11.83 1.59 42.57 5.78 0.01 0 Unburned 19.05 2.45 16.62 7.62 22.00 7.50 16.43 1.38 27.18 4.92 0.01 0 Nest 19.47 2.72 13.21 6.82 23.63 8.08 15.58 1.46 30.58 5.26 0.01 0 Random 23.92 2.72 19.25 6.82 8.38 8.08 12.68 1.49 39.17 5.26 1.78 0.48 Burned 24.10 1.88 13.97 1.54 12.85 2.20 25.88 2.31 25.72 3.13 0.76 0.38 Unburned 32.00 4.59 5.30 3.43 22.00 5.47 25.50 5.17 16.70 6.54 0.01 0 Nest 26.96 3.51 9.83 2.66 17.89 4.18 24.32 3.65 23.00 4.18 0.66 0.33 Random 29.14 3.51 9.44 2.66 16.96 4.18 27.07 3.65 19.42 4.18 1.19 0.32 68 Texas Tech University, Anthony Roberts, May 2009
Table 3.8. Lark sparrow nest site characteristics on burned and unburned mixedgrass prairie sites associated with the East Amarillo
Complex wildfires of 2006. Means are for nests on burned (n = 10) and unburned (n = 4) plots as well as all nests (n = 14) and all
random points (n = 14).
Mixedgrass Grass 2007 2008 Forbs Woody Litter Bareground Nearest Woody Mean SE Mean SE Mean SE Mean SE Mean SE Mean SE Burned 25.20 1.47 16.44 2.59 6.40 2.76 7.00 0.94 47.4 5.65 0.12 0.09 Unburned 26.50 3.29 10.50 5.79 7.30 6.16 14.00 2.09 42.5 12.63 0.01 0 Nest 23.10 2.55 13.94 4.49 8.10 4.77 13.00 1.62 41.9 9.78 0.09 0.07 Random 28.60 2.55 13.00 4.49 5.60 4.77 8.00 1.62 48 9.78 0.38 0.22 Burned 30.98 6.44 9.41 3.37 2.97 0.91 29.40 2.04 27.41 5.79 1.35 0.83 Unburned 37.84 7.87 10.68 4.13 0.33 0.91 31.00 2.64 19.65 7.11 3.40 1.72 Nest 33.37 5.33 7.96 2.80 2.60 1.11 30.03 2.36 25.91 5.44 2.12 0.84 Random 35.44 5.33 12.13 2.80 0 1.11 30.03 2.36 21.14 5.44 3.20 0.81 69