THE SOUTHWESTERN NATURALIST 54(2):136–145
JUNE 2009
NESTING SUCCESS OF GRASSLAND BIRDS IN SHINNERY OAK
COMMUNITIES TREATED WITH TEBUTHIURON AND GRAZING IN
EASTERN NEW MEXICO
LINDSAY A. SMYTHE
AND
DAVID A. HAUKOS*
Department of Natural Resources Management, Box 42125, Texas Tech University, Lubbock, TX 79409
Present address of LAS: Kofa National Wildlife Refuge, 9300 East 28th Street, Yuma, AZ 85365
*Correspondent: david.haukos@ttu.edu
ABSTRACT—Sand shinnery oak (Quercus havardii) communities, a major component of grassland
habitat of birds in eastern New Mexico, frequently are managed with livestock grazing and herbicide
application for control of shrubs. We examined nest density, nest-site selection, and daily rate of survival
of nests of grassland birds among four combinations of treatments with tebuthiuron (0.75 kg/ha) and a
short-duration, rotational-grazing system being used to restore sand shinnery oak communities in
eastern New Mexico. During breeding seasons 2004 and 2005, we searched 4-ha subplots in four
tebuthiuron-grazing-combination replicates for nests, measured vertical and overhead cover at each nest
site and an associated random point, and estimated daily rate of survival of nests using program MARK.
Density of nests was similar among all treatments but greater in 2005 than 2004. Although vertical cover
differed among treatments and between years, it did not affect selection or success of nest sites.
Overhead cover also differed among treatments and between years. Birds selected nest sites with greater
overhead cover but this did not influence success of nest, which was low in all treatments. During
incubation, daily survival of nests was greater in untreated plots; however, during the nestling period,
daily survival of nests was greater in tebuthiuron-treated plots. Neither tebuthiuron nor grazing
treatment appeared to adversely affect density of nests of grassland birds or success for the species
studied, but the low daily rate of survival of nests in this community bears further investigation.
RESUMEN—Comunidades de Quercus havardii, un componente principal del hábitat pastoral de aves en
el este de Nuevo México, han sido frecuentemente manejadas con el pastoreo de ganado y la aplicacion
de herbicidas para el control de matorral. Examinamos la densidad de nidos, el sitio seleccionado para
el nido, y la tasa de sobrevivencia diaria de nidos de aves de pastizales en cuatro combinaciones de
tratamientos con tebuthiuron (0.75 kg/ha) y manejo rotativo de pastoreo a corto plazo utilizadas para
restaurar las comunidades de Quercus havardii en el este de Nuevo México. En el perı́odo reproductivo
de 2004 y 2005, buscamos nidos en cuatro sublotes de 4 ha cada uno en cuatro réplicas de las
combinaciones de tebuthiuron y pastoreo, medimos la cubierta vertical y la que estaba sobre cada nido y
las mismas de un punto al azar, y estimamos la tasa de sobrevivencia diaria de nidos utilizando el
programa MARK. La densidad de nidos fue similar en todos los tratamientos, pero mayor en el 2005 que
en el 2004. A pesar de que la cubierta vertical difirió entre tratamientos y años, no afectó la selección del
sitio del nido ni su éxito. La cubierta sobre los nidos varió entre tratamientos y años. Las aves nidificaron
en sitios con más cubierta, pero ésto no afectó el éxito del nido, que fue bajo en todos tratamientos.
Durante la incubación, la sobrevivencia diaria de nidos fue superior en lotes no tratados; sin embargo,
durante el periodo de crı́a la sobrevivencia diaria de nidos fue superior en lotes tratados con
tebuthiuron. Aparentemente ni el tratamiento de tebuthiuron ni de pastoreo tuvieron efecto adverso ni
en la densidad de nidos de aves de pastizales ni en el éxito para las especies estudiadas, pero la baja tasa
de sobrevivencia diaria de nidos en esta comunidad llama futura investigación.
The short-grass and mid-grass prairie ecosystem
of eastern New Mexico (Partners in Flight Physiographic Area 55; http://www.partnersinflight.
org) provides habitat for many species of grassland
birds, including numerous migratory species of
concern. Data from the Breeding Bird Survey
demonstrate consistent declines across the
breeding range of most species of grassland birds
during 1966–2001 (Peterjohn and Sauer, 1999;
Sauer et al., 2001). Furthermore, grassland birds
June 2009
Smythe and Haukos—Nesting success of grassland birds
exhibited the most widespread and greatest
annual mean decline of all surveyed avian groups
(Herkert, 1995). Nest depredation may contribute to decline of grassland species or limit
recovery of populations (Basore et al., 1986),
and depredation is a leading cause of failure of
nests among grassland passerines (Martin, 1993;
Davis, 2003; Winter et al., 2004). Vertical and
horizontal cover at a nest site may influence
likelihood of depredation; nesting birds rely
mostly on decreasing detection or accessibility
for protection of nests (Martin, 1995). Thus,
land-management practices that affect vegetational structure may also impact success of nests;
e.g., Hughes et al. (2000) determined that daily
rates of survival of nests for mourning doves
(Zenaida macroura) were influenced by field-level
vegetational structure.
In New Mexico, sand shinnery oak (Quercus
havardii) communities were historically co-dominated by shrubs and grasses (Martin, 1990;
New Mexico Partners in Flight, http://www.
nmpartnersinflight.org/bcp.html). There is no
evidence that shinnery oak invades overgrazed
rangeland, but oak is an effective water-gatherer,
and when given an advantage may almost eliminate
associated plants due to effects of shading and
competition for moisture (Peterson and Boyd,
1998). Thus, unmanaged grazing can change
community composition, resulting in decreased
grass production and greater frequency of shinnery
oak (Peterson and Boyd, 1998). In many areas,
shinnery oak has become essentially a monoculture, and there is interest in restoring communities
to a more historic grass-shrub balance.
Grazing and herbicide application, common
management practices used to restore sand
shinnery oak communities, may affect nest
success through influence on available nesting
substrate. In New Mexico, the predominant
method of control of shinnery oak is application
of herbicide (Peterson and Boyd, 1998). During
1981–1993, the United States Bureau of Land
Management treated 40,469 ha of shinnery oak
in New Mexico with the herbicide tebuthiuron
(N-[5-(1,1-dimethylethyl)-1, 3, 4-thiadiazol-2-yl]N, N9-dimethylurea; United States Department of
the Interior, 1997). Depending on rate of
application, treatment with tebuthiuron tends
to decrease vertical screening immediately after
application as the shinnery oak dies, but as
bunchgrasses recover, vertical screening in treated plots may surpass that in untreated plots
137
(Doerr and Guthery, 1983). Likewise, canopy
cover eventually can be greater in tebuthiurontreated plots than in untreated plots ( Jones,
1982; Doerr and Guthery, 1983).
Birds in grassland communities evolved in
conjunction with grazing; however, effects of
grazing on bird communities and their habitats
are neither uniform nor easily defined (Wiens
and Dyer, 1975). Knopf et al. (1988) argued that
grazing has the greatest influence on nongame
birds as it alters the horizontal patterning of
lower vegetational layers; similarly, Kantrud and
Kologiski (1982) stated that grazing by domestic
livestock can alter use of grasslands by breeding
birds through changes in height and density of
vegetation. Roseberry and Klimstra (1970) detected an inverse relationship between intensity
of grazing and use of habitat by nesting eastern
meadowlarks (Sturnella magna).
In shinnery oak communities, data on nesting
success are available only for the lesser prairiechicken (Tympanuchus pallidicinctus). Riley et al.
(1992) reported that in shinnery oak and
bluestem (Andropogon and Schizachyrium) communities in southeastern New Mexico, nests were
more successful in taller vegetation. They suggested that tall, wide clumps of vegetation with
spreading stems provided better concealment
from predators, both overhead and laterally
(Riley et al., 1992). Haukos and Smith (1989)
reported that vertical screening cover and
percentage of overhead cover were the most
important features in selection of nest sites by
prairie-chickens. However, there is little information available on density, site selection, or
success of nests of passerines in shinnery oak
communities. Also, existing studies assume that
shinnery oak communities are pristine, when
historically these communities supported a
greater grass component (Peterson and Boyd,
1998). Our objective was to determine if treatment with tebuthiuron or short-duration grazing
used to restore sand shinnery oak communities
to historic shrub and grass mixes impacted
reproductive success of grassland birds. We
accomplished this by testing four combinations
of tebuthiuron and grazing treatments to determine if the subsequent vegetational response
affected density, site selection, or daily rate of
survival of nests of grassland birds.
MATERIALS AND METHODS—Our study site in Roosevelt
County, New Mexico, consisted of 16 ca. 65-ha plots
138
The Southwestern Naturalist
(one plot was ca. 80 ha). Tebuthiuron was applied at
0.75 kg/ha to 532 ha of private land in 2000, which was
adjacent to 518 ha of untreated State Game Commission-owned land (North Bluit Prairie Chicken Area)
representing the extant shinnery oak-grassland community. This rate of application rate was ,50% of
previously recommended rates to ensure that sand
shinnery oak was not completely eliminated from the
community. The control area had not been grazed for
$7 years before the study began; tebuthiuron-treated
areas had not been grazed for $5 years before the
study began, 2 years pre-tebuthiuron treatment and 3
years post-tebuthiuron treatment. Grazing treatment
was a short-duration system in which plots were grazed
once during the dormant season ( January and
February) and once during the growing season ( July).
Stocking rate was calculated each season based on
measured forage production and designed to take 25%
of available herbaceous material per season. During
2003–2005, stocking rate was 584–2,224 animal-days on
the tebuthiuron-treated plots and 147–556 animal-days
on the untreated plots (Smythe, 2006). Higher rates of
stocking occurred during the dormant season, while
lower rates occurred during the growing season. Plots
consisted of two treatments arranged in four combinations: tebuthiuron with grazing, tebuthiuron without
grazing, no tebuthiuron with grazing, and a control of
no tebuthiuron or current grazing. Average yearly
precipitation in the region is 31.5 cm (United States
Department of Commerce, http://www.ncdc.noaa.
gov/oa/climate/research/cag3/z3.html); however, 2003
represented the end of a 15-year drought at
26.15 cm of precipitation for the year in the study
area. In 2004, cumulative precipitation was 85.85 cm,
the second highest ever recorded in the region.
Precipitation in 2005 was closer to average at
27.5 cm (C. Dixon, unpublished data).
In each of the 16 treatment plots (four replications
of four treatment combinations), we randomly placed
four 4-ha subplots. We conducted searches for nests on
each plot in March, April, May, and June 2004. Because
no nest was found in March 2004, we searched for nests
only in April, May, and June 2005. We randomly
selected two subplots in each treatment plot to be
searched each month, but we were limited by time and
available labor to searching only one-half of each
subplot. Each search consisted of walking through
either the east or west one-half of the subplot with 2–4
people, for a total of 4 ha searched/treatment replicate/month. Searches and monitoring of nests were
conducted within the guidelines of Winter et al.
(2003). Nests were located by systematically parting
vegetation with a hockey stick (Berthelsen and Smith,
1995; Koford, 1999) and watching for flushed adults.
Baxter and Wolfe (1973) reported that 94% of dummy
nests were found when searching plots with hockey
sticks.
We found active and inactive nests during searches.
Active nests usually were found by flushing an adult;
these nests contained eggshells, eggs, or young.
Inactive nests also were discovered while searching;
these nests did not contain eggs or young, had no
associated adult, and were likely $1 year old. Although
we had no way of determining true age of inactive
nests, we recorded the number found. At each active
vol. 54, no. 2
nest, we recorded GPS coordinates, identified species
of plant in which the nest was located, marked the nest
with a flag, and rechecked each nest every 4 days to
determine fate of nest (Ralph et al., 1993). At failed
nests, we checked for evidence of depredation and
identified cause of failure if possible. Determining type
of predator from evidence left at the nest can be
unreliable (Marini and Melo, 1998; Pietz and Granfors,
2000), so we did not categorize depredations. We
calculated density of nests by dividing number of nests
(active or inactive) found during all searches by total
size of area searched. March was excluded from
calculations of density in 2004 because no nest was
found.
Success of nests was calculated using the Mayfield
(1961, 1975) approach with the maximum-likelihood
method in program MARK (White and Burnham,
1999; Williams et al., 2002). We considered nests of
altricial species to be successful if $1 young fledged,
whereas nests of precocial species were considered
successful when $1 egg hatched. Daily rates of survival
were modeled among species and treatments using
program MARK. We selected the best supported model
using Akaike’s Information Criterion for selection of
model adjusted for small samples (AICc; Anderson and
Burnham, 2002). We made pairwise comparisons of
daily rates of survival between 2004 and 2005 and
between incubation and the nestling period with a x2
test using program CONTRAST as described in Sauer
and Williams (1989). We compared density of nests
among treatments and years using analysis of variance
(ANOVA) in a mixed-linear model after data were
tested for normality and heterogeneous variances
(Cochran and Cox, 1957). Treatment (tebuthiuron
and grazing combination) and year were analyzed as
fixed effects. We separated means with pairwise
comparisons of least-squares means using the leastsignificant-difference (LSD) test if the F-test on
marginal means was significant (P , 0.05).
To determine if birds were selecting specific structural characteristics at nest sites, we measured vertical
density and overhead cover at each nest site and
compared these measurements to those made at a
random point within 50 m of the nest site. We
measured vertical density by taking profile-board
readings from each cardinal direction (Nudds, 1977;
Guthery et al., 1981). The profile board consisted of 10,
10-cm strata. We recorded percentage of each stratum
obscured by vegetation and maximum height of
vegetation. Readings were taken at a distance of 7 m
from the nest site at a height of 1 m (Guthery et al.,
1981). We took digital photographs of each nest from a
height of 1 m and then quantified concealment of the
nest by using a photo software program to outline the
nest and divide the number of vegetational pixels by
number of total pixels of nest (Ortega et al., 2002). We
compared these measurements to random points by
importing the outline of each nest into a corresponding photo of a random point and dividing the number
of vegetation pixels by the same total number of
corresponding pixels of nests.
Comparison of vertical structure in different strata
between nest sites and random points for vertical cover
was performed using multivariate analysis of variance
(MANOVA; Manly, 1994). Overhead cover between
June 2009
Smythe and Haukos—Nesting success of grassland birds
139
TABLE 1—Total number of nests of birds and average density (6SE) among four combinations of tebuthiuron
treatment and grazing in sand shinnery oak (Quercus havardii) communities in Roosevelt County, New
Mexico, 2004–2005.
Number of active nests Number of inactive nests
Treatment
2004
2005
2004
2005
Untreated, grazed
Untreated, ungrazed
Treated, grazed
Treated, ungrazed
5
7
3
11
10
15
15
16
15
13
34
35
9
6
31
24
a
b
c
Density of active nests (nests/10 ha)
2004
0.21
0.83
0.21
1.46
6
6
6
6
0.21
0.48
0.21
0.63
a
2005
Abac
Aa
Aa
Aa
1.25
1.46
2.08
2.71
6
6
6
6
0.24
0.93
0.87
0.40
Ab
Ab
Ab
Ab
Density includes only nests found in subplots during searches.
Uppercase letters indicate differences (P , 0.05) among treatments within a year.
Lowercase letters indicate differences (P , 0.05) between years within a treatment.
nest and random sites was analyzed as a completely
randomized design in a mixed-linear model in ANOVA
(Cochran and Cox, 1957). Treatment (tebuthiuron
and grazing combination), hatching success (hatch or
fail), and year were analyzed as fixed effects. We
separated means using a LSD-test if the overall F-test
was significant (P , 0.05). Statistical analyses were
performed using SAS statistical software (SAS Institute,
Inc., 2003).
RESULTS—During March–June 2004, 256 ha
were searched for nests (64 ha/month). Across
all treatments in 2004, 26 active nests and 97
inactive nests of various species were found
(Smythe, 2006). During April–June 2005,
192 ha were searched (64 ha/month). Across
all treatments in 2005, we found 59 active nests
and 72 inactive nests of various species (Smythe,
2006). The first nest was found 17 days earlier in
2005 (1 April) than in 2004 (18 April). Density of
nests was calculated using only nests found in
subplots during searches. Nests found incidentally outside of searches were used in all other
comparisons.
Density of Nests—Density of nests (nests/10 ha)
for all species was similar among treatments
(F3,24 5 2.07, P 5 0.13), but density of nests in
2005 was more than twice that in 2004 (mean for
2004 5 0.70 6 0.20 SE, mean for 2005 5 1.90 6
0.30; F1,24 5 9.06, P , 0.01; Table 1). Cassin’s
sparrows (Aimophila cassinii), mourning doves
(Zenaida macroura), lesser prairie-chickens, and
meadowlarks (Sturnella) were most common
(Smythe, 2006). These species were categorized
as resident or migrant based on whether they
were present on the study site year-round or only
during the breeding season. Cassin’s sparrows
and mourning doves were migrants and meadowlarks and lesser prairie-chickens were resi-
dents. Density of nests of Cassin’s sparrows
(nests/10 ha) was not affected by treatment
(F3,24 5 1.32, P 5 0.29), but density was six times
greater in 2005 than 2004 (mean for 2004 5 0.20
6 0.10, mean for 2005 5 1.20 6 0.20; F1,24 5
13.44, P , 0.01). We found too few nests of other
species to make comparisons of densities among
treatments or years.
Success of Nests—Because there was no difference
in daily rates of survival for any species between
2004 and 2005 (Smythe, 2006), we combined data
from both years for further analysis. Daily rate of
survival across treatments did not differ between
incubation and nestling period for any species or
within any treatment (Table 2).
Across treatments, models with the lowest
AICc-values indicated that daily rates of survival
were different among species during incubation
but not during the nestling period (Table 3).
Among treatments, daily rates of survival differed
during both incubation and the nestling period
(Table 4). During incubation, the model with
the lowest AICc indicated that daily survival of
nests differed between tebuthiuron-treated and
untreated plots (Table 4); survival was 6.3%
higher in untreated plots than in treated plots
(Table 2). During the nestling period, the same
model indicating that daily survival of nests was
the same among tebuthiuron-treated plots and
among untreated plots was slightly less supported by data as the model with the lowest AICc, but
we chose this model because it was more logical
in regard to effects of treatment than the model
with the lowest AICc (Table 4). However, during
the nestling period the opposite trend manifested; daily survival of nests was 17.3% higher in
tebuthiuron-treated plots than in untreated plots
(Table 2).
140
vol. 54, no. 2
The Southwestern Naturalist
TABLE 2—Daily rates of survival of nests during incubation and nestling period for four species of grassland birds
and among restoration treatments in sand shinnery oak (Quercus havardii) communities in Roosevelt County, New
Mexico, 2004–2005.
Incubation (n)
Hatching to fledging (n)
By species
All species
Cassin’s sparrow
Lesser prairie-chicken
Meadowlark
Mourning dove
0.915
0.892
0.963
0.926
0.864
6
6
6
6
6
0.013
0.023
0.018
0.036
0.039
(64) aa
(30) Aba
(8) B
(7) Ca
(16) Da
0.962
0.922
0.916
0.856
6
6
6
6
0.018
0.023
0.026
0.035
(11)
(18)
(15)
(20)
0.854 6 0.034
0.780 6 0.067
N/A
0.933 6 0.046
0.723 6 0.126
(19) a
(10) Aa
(3) Aa
(4) Aa
By treatment
Untreated, grazed
Untreated, ungrazed
Treated, grazed
Treated, ungrazed
Acad
Aa
Ba
Ba
0.688
0.805
0.939
0.816
6
6
6
6
0.122
0.106
0.034
0.076
(4)
(4)
(5)
(5)
Aa
Aa
Ba
Ba
a
Lowercase letters indicate differences (P , 0.05) between incubation and nestling period within species.
Uppercase letters indicate differences (based on MARK models) within incubation or nestling period among species.
c Uppercase letters indicate differences (based on MARK models) within incubation or nestling period among
treatments.
d Lowercase letters indicate differences (P , 0.05) between incubation and nestling period within a treatment.
b
TABLE 3—Comparison of models for daily rates of survival using program MARK among species of birds across
treatments in sand shinnery oak (Quercus havardii) communities in Roosevelt County, New Mexico, during April–
June 2004 and 2005.
Model
AICc
D AICc
Number of parameters
Deviance
160.402
161.586
0.00
1.18
4
1
152.294
159.576
29.233
31.276
0.00
2.04
1
2
27.131
26.960
Incubation among species
A – all species different
B – all species same
Nestling period among species
B – all species same
A – all species different
TABLE 4—Comparison of models for daily rates of survival of birds using program MARK among treatments across
species in sand shinnery oak (Quercus havardii) communities in Roosevelt County, New Mexico, April–June 2004 and 2005.
Model
Incubation among
AICc
D AICc
Number of parameters
Deviance
171.112
171.437
172.200
173.056
174.947
0.00
0.33
1.09
1.94
3.84
2
4
2
1
2
167.083
163.341
168.171
171.046
170.919
51.698
51.892
53.071
53.391
54.604
0.00
0.19
1.37
1.69
2.91
2
2
4
1
2
47.562
47.756
44.606
51.346
50.467
treatmentsa
A – treatments 1 and 2 same, 3 and 4 same
B – all treatments different
C – treatments 1 and 3 same, 2 and 4 same
D – all treatments same
E – treatments 1 and 4 same, 2 and 3 same
Nestling period among treatments
E – treatments 1 and 4 same, 2 and 3 same
A – treatments 1 and 2 same, 3 and 4 same
B – all treatments different
D – all treatments same
C – treatments 1 and 3 same, 2 and 4 same
a
Treatment 1 5 untreated, grazed; 2 5 untreated, ungrazed; 3 5 treated, grazed; 4 5 treated, ungrazed.
June 2009
Smythe and Haukos—Nesting success of grassland birds
141
TABLE 5—Means of visual obscurity and maximum height by treatment at nests of 66 grassland birds and
associated random points in sand shinnery oak (Quercus havardii) communities in Roosevelt County, New
Mexico, 2004–2005.
Visual obscurity (%)
Stratum
1
2
3
4
5
6
7
8
9
10
Maximum height (cm)
a
cm
0–10
11–20
21–30
31–40
41–50
51–60
61–70
71–80
81–90
91–100
Untreated,
grazed
94.1
87.3
65.2
41.9
23.3
13.6
6.8
4.3
2.0
1.6
64.1
6
6
6
6
6
6
6
6
6
6
6
3.9
4.3
4.7
4.9
4.6
2.8
1.5
1.0
0.5
0.6
4.5
aa
a
a
a
c
b
c
c
a
a
c
Untreated,
ungrazed
96.4
89.8
67.6
51.7
27.9
17.1
9.7
6.2
3.1
2.1
72.4
6
6
6
6
6
6
6
6
6
6
6
1.0
2.0
4.0
8.8
2.8
2.5
1.6
1.0
0.6
0.4
3.5
a
a
a
a
bc
b
c
bc
a
a
b
Treated,
grazed
96.2
85.5
59.2
49.0
36.8
31.5
18.3
11.1
5.1
3.7
79.5
6
6
6
6
6
6
6
6
6
6
6
Treated,
ungrazed
1.6 a
3.0 a
3.4 a
3.6 a
3.7 ab
23.7 a
2.6 b
1.9 b
1.2 a
1.0 a
2.5 a
97.7
89.2
69.6
59.1
44.6
37.0
26.5
19.6
10.2
6.5
79.2
6
6
6
6
6
6
6
6
6
6
6
0.7
1.7
2.8
3.2
3.9
3.9
3.6
3.1
2.4
2.1
2.6
a
a
a
a
a
a
a
a
a
a
a
Lowercase letters indicate differences (P , 0.05) among treatments within a stratum.
Only four nests were considered successful in
2004: one meadowlark, two Chihuahuan ravens
(Corvus cryptoleucus), and one mallard (Anas
platyrhynchos). In 2005, six nests were successful:
one Chihuahuan raven, one northern bobwhite
(Colinus virginianus), one Cassin’s sparrow, and
three lesser prairie-chickens. Depredation accounted for 84% of all failures of nests.
Vegetation at Nest Sites—We were able to collect
vegetational-composition data for 73 of the 85
active nests. The majority of nests of Cassin’s
sparrow (76%) and meadowlarks (90%) were in
little bluestem (Schizachyrium scoparium; Smythe,
2006). We found too few nests of other species to
make categorizations, although sand sagebrush
(Artemisia filifolia) appeared to be another
important nesting substrate. All nests found in
sand sagebrush were in untreated plots; the
tebuthiuron treatment removed all sand sagebrush in treated plots.
We analyzed profile-board data from 66 active
nests and 66 associated random points. We
excluded nests in trees, nests found by other
researchers that we did not personally monitor,
and nests that already had failed when found.
Although there was a difference in vegetational
structure at sampling points (nest and random)
among treatments (Wilks’ l 5 0.54, P , 0.01) and
between years (Wilks’ l 5 0.38, P , 0.01), there
was no difference in vegetational structure between nest and random sites (Wilks’ l 5 0.87, P 5
0.17). Among treatments, visual obscurity differed
only in strata 5–8 (41–80 cm) and maximum
height (Table 5). In these strata, the treated,
ungrazed plots consistently had the greatest visual
obscurity, whereas untreated, grazed plots had the
lowest (Table 5). Vegetation was tallest in tebuthiuron-treated plots (Table 5). Visual obscurity was greater in these strata in 2005 and
maximum height was nearly twice that in 2004
(Table 6). Across years, vegetational structure at
TABLE 6—Means of visual obscurity and maximum
height by year at nests of 66 grassland birds and
associated random points in sand shinnery oak
(Quercus havardii) communities in Roosevelt County,
New Mexico, 2004–2005.
Visual obscurity (%)
Stratum
1
2
3
4
5
6
7
8
9
10
cm
0–10
11–20
21–30
31–40
41–50
51–60
61–70
71–80
81–90
91–100
Maximum height (cm)
a
2004
96.7
87.2
64.9
45.4
27.3
18.0
9.6
5.6
3.5
3.3
6
6
6
6
6
6
6
6
6
6
1.5
3.0
4.3
4.9
5.5
4.9
3.7
3.0
2.7
2.7
2005
aa
a
a
a
a
a
a
a
a
a
50.0 6 3.0 a
96.1
88.4
66.1
53.5
36.8
28.9
18.9
13.2
6.4
4.0
6
6
6
6
6
6
6
6
6
6
1.1
1.4
2.0
3.2
2.0
2.0
1.6
1.4
0.9
0.5
a
a
a
a
b
b
b
b
a
a
82.5 6 1.2 b
Lowercase letters indicate differences (P , 0.05)
between years within a stratum.
142
The Southwestern Naturalist
hatched nests did not differ from vegetation at
failed nests (Wilks’ l 5 0.91, P 5 0.92) regardless
of treatment (Wilks’ l 5 0.63, P 5 0.85).
Across treatments, vertical structure did not
differ between hatched and failed nests in either
2004 (Wilks’ l 5 0.07, P 5 0.07) or 2005 (Wilks’
l 5 0.83, P 5 0.73). We were unable to test for
an interaction between hatching success, treatment, and year because of the lack of nests that
hatched in certain treatment-year combinations.
We analyzed overhead-cover data from 55
nests of grassland birds and 55 associated
random points. Meadowlark nests were excluded
because they modify the nest site by pulling grass
over the top. Overhead cover was greater at nest
sites than at random sites (n 5 110; mean of nest
sites 5 72.8 6 3.3%, mean of random sites 5
44.0 6 4.5%; F 1,96 5 15.41, P , 0.01). This
relationship was true in both years (F 1,96 5 0.0, P
5 0.98), regardless of treatment (F 3,96 5 1.74, P
5 0.16). Overhead cover at all sample points
(nest and random) did not differ among
treatments (F 3,96 5 0.92, P 5 0.43), although
it was greater in 2005 than 2004 (mean for 2004
5 41.0 6 7.6%, mean for 2005 5 62.2 6 3.3%; F
1,96 5 7.49, P , 0.01). There was no difference in
overhead cover between nests that hatched and
nests that did not hatch (n 5 56; mean number
that hatched 5 70.6 6 5.7%, mean number that
failed 5 74.2 6 3.8%; F 1,41 5 1.86, P 5 0.18),
regardless of treatment (F 3,415 0.78, P 5 0.51).
Average overhead cover for all open-cup and
ground-nesting species was 72.8 6 3.2% (n 5
56). Average overhead cover at nests of Cassin’s
sparrows was 74.8 6 3.8% (n 5 34). Average
overhead cover at nest sites of mourning doves
was 73.8 6 6.0% (n 5 16).
DISCUSSION—Use of tebuthiuron and grazing
management to restore shinnery oak communities had little effect on initial density of nests of
grassland birds. Despite greater avian density in
tebuthiuron-treated areas (Smythe, 2006), density of nests was similar among treatments; i.e.,
more birds in a treatment did not translate to
more nests. Overall density of nests was greater
in 2005 than 2004, and the effect was most
pronounced in migrant species. Equal numbers
of nests of residents (lesser prairie-chicken,
meadowlark) were found in both years, in
contrast to the more numerous nests of migrants
(Cassin’s sparrow, mourning dove) found in
2005 than 2004. Migrant species appeared to
vol. 54, no. 2
respond to improved conditions of vegetation in
2005 by initiating more nests. Resident species
did not initiate more nests but had higher rates
of survival. The first nest was found 17 days
earlier in 2005 than in 2004, possibly because of
better initial conditions in 2005 than 2004.
Precipitation likely had an important influence on results. This study was conducted over
years of unusual precipitation in the region;
beginning in drought conditions in 2003, followed by twice the average yearly precipitation in
2004 (C. Dixon, unpublished data). These
precipitation patterns created atypical vegetational conditions throughout the study site, and
these results may not hold in dry-to-average
years.
Grassland birds did not exhibit selection
among nest sites based on vertical density, nor
did vertical density affect hatching success.
However, differences in vertical density among
treatments and years occurred only at heights
.40 cm. At lower levels of vegetation, those most
important for concealment of nests, there was no
difference in vertical density among treatments
and no need for birds to select nest sites. Average
height of shinnery oak on the study site was
46.4 cm (C. Dixon, unpublished data). This
indicates that at lower vegetational strata, untreated shinnery oak provides similar vertical
screening as the predominantly little bluestem
communities that replace them after treatment
with tebuthiuron. In this study, the moderate
grazing regime did not significantly impact
vertical density.
Grassland birds selected nest sites based on
overhead cover, presumably as a defense against
avian predators. Although average overhead
cover did not differ among treatments, and
ranged from 41 to 61% from 2004 to 2005,
nesting birds selected sites with 72% cover on
average. Overhead cover was greater at nest sites
than at random points, but this did not influence
whether or not the nest hatched.
Greater horizontal and vertical cover in 2005
versus 2004 did not translate to higher daily rates
of survival of nests. Likewise, greater vertical
cover in tebuthiuron-treated plots did not always
result in higher daily rates of survival of nests; in
fact, untreated plots had higher daily rates of
survival of nests during incubation, although
tebuthiuron-treated plots had higher rates during the nestling period. This may indicate that
grasses and shrubs are needed during different
June 2009
Smythe and Haukos—Nesting success of grassland birds
periods of brood rearing and, thus, both are
required in a restored shinnery oak community.
Several other studies documented a similar
disassociation between success of nest and
vegetation. Newton and Heske (2001) detected
no relationship between vertical density in a field
and number of artificial nests depredated in that
field; Rivers et al. (2003) reported no consistent
difference in side and overhead concealment
between depredated and undisturbed artificial
nests. Winter et al. (2005) did not find recognizable effects of vegetational structure on
nesting success in two of three grassland species,
and Kershner et al. (2004) detected no difference in nest-site vegetational characteristics
between successful and failed nests of meadowlarks. The inconsistent effects of concealment of
nests may be related to ecology of predators;
medium-sized predators (e.g., striped skunk
Mephitis mephitis; Chihuahuan raven) may be less
likely to find a well-concealed nest, but small
predators (e.g., hispid cotton rat Sigmodon
hispidus; spotted ground squirrel Spermophilus
spilosoma) may be more likely to depredate a
well-concealed nest if the concealment offers
protection from secondary predators (Dion et
al., 2000; Rivers et al., 2003). Losses of nests to
predation were high, but this has been documented in other grassland birds (Martin, 1993;
Davis, 2003). In addition to improving habitat
for grassland birds, the abundant rainfall may
have improved conditions for predators of
grassland birds.
Resident birds (lesser prairie-chicken, meadowlark) had higher daily rates of survival during
incubation than did migrants (Cassin’s sparrow,
mourning dove). However, all species had low
daily rates of survival from hatching to fledging.
We did not find a difference in daily rates of
survival between incubation and nestling periods, although our nestling-period calculations
were based on a relatively small sample. Stake
and Cimprich (2003) documented more-frequent predation during the nestling stage than
during incubation. Predation tends to be higher
during the nestling stage than during incubation, possibly because noise, movement, and
scent of nestlings attracts predators (Eichholtz
and Koenig, 1992), although Davis (2003) did
not detect differences between incubation and
rates of survival of nestlings of prairie songbirds.
With the exception of the lesser prairiechicken, rates of nest success for all species to
143
fledging were low compared to those of several
other studies in grasslands (Lanyon, 1957;
Roseberry and Klimstra, 1970; Haukos, 1988;
Riley et al., 1992; Berthelsen and Smith, 1995;
Hughes et al., 2000; Kershner et al., 2004). This
is discouraging considering the optimal conditions during the study of light grazing and
abundant rainfall. Low rate of success of nests
may indicate that overall quality of nesting
habitat is low ( Johnson and Temple, 1986), but
does not necessarily translate to reduced annual
productivity, as females may compensate by
renesting and double-brooding (Murray, 2000).
Powell et al. (1999) theorized that small increases in daily rates of survival of adults and juveniles
could have a large positive effect on breedingseason productivity. In spite of preventative
efforts (Winter et al., 2003), it was possible that
disturbance by humans increased predation on
monitored nests. However, the low rate of
success of nests suggests the possibility that this
community may currently be a population sink
or ecological trap.
Restoring shinnery oak communities using
tebuthiuron and grazing treatments clearly
altered vegetational structure, but impacts of
these treatments on avian reproduction remain
unclear. The above-average rainfall may have
masked some effects of treatment; birds may be
more selective in dry years. It also is important to
consider that control areas did not represent
shinnery oak communities in pristine condition,
and there may also be a lag in reproductive
response of birds to treatments. Our results
indicate that carefully managed application of
tebuthiuron and grazing in shinnery oak communities do not adversely impact density or
success of nests of grassland birds; however,
current high rates of depredation and low rates
of nest success overall do not bode well for
grassland birds in this community.
We thank C. Dixon of Wildlife Plus Consulting for
sharing data on vegetation and rainfall, and M. Patten
of the G. M. Sutton Avian Research Center for sharing
data on nests of lesser prairie-chickens. We also thank
the hardworking field technicians who assisted with
data collection: A. Andrei, K. Bailey, H. Coin, D. Ferris,
J. Hull, B. Rigby, D. Strain, and P. Whiting. P. McDaniel
of Phalarope Consulting also provided assistance on
this study. B. Vizcarra and C. Vigil translated the
abstract into Spanish. We thank the anonymous
reviewer who provided comments to improve the
manuscript. This study was funded by a grant from
144
The Southwestern Naturalist
the National Fish and Wildlife Foundation, with
support from the Region 2 Migratory Bird Office of
the United States Fish and Wildlife Service, the
Department of Natural Resources Management, Texas
Tech University, and Grasslands Charitable Foundation. This is manuscript T-9-11143 of the College of
Agricultural Sciences and Natural Resources, Texas
Tech University.
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