Response of Grassland Birds in Sand Shinnery Oak
Communities Restored Using Tebuthiuron and
Grazing in Eastern New Mexico
Lindsay A. Smythe1,2,3 and David A. Haukos1
Abstract
Sand shinnery oak (Quercus havardii) communities are
a unique component of grassland bird habitat in eastern
New Mexico and have been impacted by human activities
for decades. These communities are frequently managed
with livestock grazing and herbicide application for shrub
control, strategies that potentially can be used to restore
the historical shrub–grass composition of this plant community. During spring migration and the breeding seasons
of 2004 and 2005, we compared density and community
structure of grassland bird species among four combinations of tebuthiuron application and grazing treatments
that were being evaluated for restoration of shinnery oak
communities. We performed biweekly point transects
on sixteen 65-ha study plots in these communities. Density
of all avian species combined did not differ between
grazed and ungrazed plots. Tebuthiuron-treated plots had
a 40% higher average density for combined species than
untreated plots. There was a 41% higher average density
of all species during spring 2005 than 2004, but density
was similar during the breeding season of both years.
These trends were predominantly influenced by densities
of migratory Cassin’s Sparrow (Aimophila cassinii),
which were greater in tebuthiuron-treated plots in both
years. Densities of resident Meadowlarks (Sturnella spp.)
exhibited little response to tebuthiuron or grazing treatments. Avian species richness, evenness, and diversity
were only minimally affected by the tebuthiuron and grazing treatments. This study occurred over a period of highly
variable precipitation, so future assessments, spanning
longer wet–dry cycles and maturing plant communities,
may be necessary to completely determine avian response
to these restoration efforts.
Introduction
The short- and mixed-grass prairie ecosystem of eastern
New Mexico provides habitat for many species of grassland
birds, including numerous migratory species of concern
(Rich et al. 2004; NMPIF 2007). Data from the North
American Breeding Bird Survey demonstrate consistent
declines across the breeding range of most grassland bird
species between 1966 and 2005 (Peterjohn & Sauer 1999;
Sauer et al. 2005). Furthermore, populations of grassland
birds showed the most widespread and greatest decline of
all surveyed avian groups (Herkert 1995). Habitat destruction, fragmentation, and degradation appear to be the
underlying influences in these population trends, although
these effects may manifest differently among species
(Peterjohn & Sauer 1999). Grassland birds tend to select
habitats based on vegetation structure (Wiens 1973), and
thus modification of grassland habitat may affect the abundance and distribution of bird populations (Martin 1995).
Sand shinnery oak (Quercus havardii) communities are
a unique vegetation type interspersed in the more traditionally defined grasslands of eastern New Mexico, exemplified
by shrubs such as Sand sage (Artemisia filifolia) and Sand
shinnery oak, as well as several grasses including Sand bluestem (Andropogon hallii), Big bluestem (An. gerardii), Purple three-awn (Aristida purpurea), Hairy grama (Bouteloua
hirsuta), Fall witchgrass (Leptoloma cognatum), Little bluestem (Schizachyrium scoparium), and Sand dropseed
(Sporobolus cryptandrus) (Martin 1990; NMPIF 2007; plant
names follow Hitchcock 1971). These communities were
historically codominated by shrubs and grasses (Martin
1990; NMPIF 2007). There is no evidence that shinnery oak
invades overgrazed rangeland, but this species is an effective water gatherer and, when given an advantage, may
nearly eliminate associated plants due to the effects of
shading and moisture competition (Peterson & Boyd 1998).
Thus, unmanaged grazing can change the community composition, resulting in decreased grass production and
greater frequency of shinnery oak (Peterson & Boyd 1998).
In many areas, shinnery oak has become dominant or
nearly a monoculture, and there is interest in restoring the
communities to a more historic grass/shrub balance.
Tools available for restoring shinnery communities to
their historical vegetation composition and structure
1
Department of Natural Resources Management, Texas Tech University, Box
42125, Lubbock, TX 79409, U.S.A.
2
Address correspondence to L. A. Smythe, email lindsay_smythe@fws.gov
3
Present address: Kofa National Wildlife Refuge, 9300 E. 28th street, Yuma,
AZ 85365, U.S.A.
Ó 2008 Society for Ecological Restoration International
doi: 10.1111/j.1526-100X.2008.00443.x
Restoration Ecology
Key words: density, DISTANCE, grassland birds, grazing,
herbicide, shinnery oak.
1
Response of Grassland Birds in Sand Shinnery Oak Communities
include shinnery oak control and managed livestock grazing. In New Mexico, the predominant method of shinnery
oak control is herbicide application (Peterson & Boyd
1998). Between 1981 and 1993, the U.S. 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)
(USDI BLM 1997). Although grassland bird communities
evolved in conjunction with seasonal ungulate grazing, the
effects of cattle grazing systems on bird communities
and their habitats are neither uniform nor easily defined
(Wiens & Dyer 1975).
According to Peterson and Boyd (1998), about 20 songbird species nest in Sand shinnery oak communities and
approximately 80 additional species historically used this
habitat, but none are endemic to the community. NMPIF
(2007) has identified eight bird priority species of concern
using Sand shinnery oak communities. These communities
have as much value for their support of prairie breeding
species during migration and winter as they do for birds
that breed in shinnery oak communities (Rich et al. 2004).
Thus, use of grazing and herbicides to restore shinnery
habitats may impact migratory bird populations throughout the southwestern United States. Also, many previous studies assumed that shinnery oak communities were
pristine, when historically these communities supported
a greater grass component (Peterson & Boyd 1998). Our
objective was to compare density, richness, evenness, and
diversity of breeding grassland bird species among four
combinations of herbicide and grazing treatments to
determine if any of these treatments negatively impacted
grassland bird community parameters.
Methods
Study Area
Our study site was in Roosevelt County, eastern New
Mexico, U.S.A., centered at lat 33.66°N, long 2103.13°W,
NAD27. The study area consisted of 16 plots of approximately 65 ha each (one plot was 80 ha). Tebuthiuron
(0.75 kg/ha) was applied aerially to 532 ha of private land
in 2000, which was adjacent to 518 ha of the state-owned
North Bluit Prairie Chicken Area that was not treated
and represented the extant shinnery oak/grassland community. A year after the tebuthiuron application, a 6.5fold increase in herbaceous plant production and a 29-fold
increase in grass seed production was recorded on the
treated areas (Dixon, unpublished data). The control area
had not been grazed for at least 7 years before the study
began; the tebuthiuron-treated areas had not been grazed
for at least 5 years before the study began, 2 years pretebuthiuron treatment, and 3 years posttebuthiuron treatment. In 2003, the treated and untreated areas were crossfenced into eight plots each, and managed grazing was
applied randomly to half of the tebuthiuron-treated and
2
untreated areas. The 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 the available herbaceous material per season. From 2003 to 2005, stocking rate ranged
from 584 to 2,224 animal-days on the tebuthiuron-treated
plots and from 147 to 556 animal-days on the untreated
plots (Smythe 2006). Higher stocking rates occurred during the dormant season. Study 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 grazing
(Smythe 2006).
Point Transects
In each of the 16 treatment plots (four replications of four
treatment combinations), we randomly placed four 4-ha
subplots. Point transects for avian sampling were set up in
a grid of four points, 100 m apart, per subplot (Ralph et al.
1993; Buckland et al. 2001). To estimate avian density in
each treatment combination, we surveyed at least two of
the four points in each subplot every month (three per
subplot were surveyed in February to ensure adequate
detections for distance sampling). Points in the same grid
were not sampled on the same day to maintain independence. We conducted point transects from February
through July of 2004 and 2005 to record spring migrant
species and breeding birds. From February through April,
we conducted point transects from sunrise until four were
completed (approximately 0600–1000); from May through
July, we conducted transects from sunrise until two were
completed (approximately 0445–0800) because the birds
became inactive more quickly as the weather grew warmer.
This sampling design differed slightly from the recommended survey period (May–July) and time frame (0500–
0900) for stable detectability of breeding birds (Ralph
et al. 1993), but distance sampling provides a reliable estimate of density in the face of variability in detection due
to factors such as cue production (e.g., singing) and environmental differences (Buckland et al. 2001). We
recorded all avian species seen or heard within 5 minutes
at the point. To reliably estimate absolute density, we
recorded distance to each bird seen in the point transect
with a Leica laser rangefinder (Ransom & Pinchak 2003).
Birds that were heard but not seen were not included in
density calculations. Density was reported as individuals
per hectare for all species except Grasshopper Sparrows
(Ammodramus savannarum) and Cassin’s Sparrows (Aimophila cassinii), which were reported as singing males per
hectare because generally only singing males were detected
during surveys. We grouped Eastern Meadowlarks
(Sturnella magna) and Western Meadowlarks (S. neglecta)
for analysis due to difficulty in identifying nonsinging individuals by sight in the field.
Restoration Ecology
Response of Grassland Birds in Sand Shinnery Oak Communities
Statistical Analyses
Bird density was estimated with program DISTANCE
(Buckland et al. 2001; Thomas et al. 2003). The detection
function model with the lowest AICc value (Akaike’s
information criterion adjusted for small sample sizes) was
selected as the best fit (Anderson & Burnham 2002). Density of each species was estimated for each month.
The tebuthiuron-treated and untreated areas were analyzed as combined completely randomized designs to
allow for inference beyond the study site (Cochran & Cox
1957). Densities were compared among treatments,
months, and years using analysis of variance in a mixed
linear model after the data were tested for normality and
homogeneous variances (Cochran & Cox 1957). Tebuthiuron treatment, grazing occurrence, month, and year
were analyzed as fixed effects, with year as the first
repeated measure and month as the second repeated measure. We separated means using the least significant difference test with pairwise comparisons of least squares
means if the F test on marginal means was significant (p <
0.05). Regression analysis of density by month was conducted using contrast coefficients within the model. We
performed statistical analyses using SAS 9.1 (SAS Institute Inc. 2003). Means are reported ± SE.
Species richness was the number of species observed
during a survey, excluding incidentals such as birds on fencelines or flying over the survey area. We used relative
abundance of bird species to calculate
diversity with the
P
Shannon–Weiner index (H9 ¼ pi ln pi ; Magurran 1988:
35) and then calculated evenness as the ratio of observed
diversity to maximum diversity (E ¼ H9/Hmax; Magurran
1988:37).
F[1,12] ¼ 11.95, p ¼ 0.005). Average total density was 41%
greater in 2005 than in 2004 (0.21 ± 0.01 vs. 0.15 ± 0.01,
respectively), but because of an interaction between
month and year (F[5,132] ¼ 3.19, p ¼ 0.01), the increase in
density from 2004 to 2005 was not consistent across months.
Density was greater in 2005 from March through May and
then converged to 2004 levels for June to July (Fig. 1).
Regression analysis of bird density by month revealed
a quadratic relationship in both years for both tebuthiuron-treated (2004: F[1,60] ¼ 10.0, p ¼ 0.003; 2005:
F[1,60] ¼ 47.42, p < 0.0001) and untreated plots (2004:
F[1,60] ¼ 16.60, p ¼ 0.0001; 2005: F[1,60] ¼ 30.43, p < 0.0001;
Fig. 1). On all plots, in both 2004 and 2005, the highest
density of birds (0.24 ± 0.03 and 0.37 ± 0.04, respectively)
occurred in April (Fig. 1).
Density by Species
Cassin’s Sparrow and Meadowlark were the only bird species for which we were able to estimate species-level densities in all survey months during both years. The study area
is located in the boundary region between wintering and
Results
This study occurred during a period of unusually variable
precipitation. Average yearly precipitation in the region is
31.5 cm (USDC NCDC 2005); however, 2003 represented
the end of a 15-year period of drought at 26.2 cm of precipitation for the year in the study area. In 2004, cumulative
precipitation was 85.9 cm, the second highest ever recorded
in this region. Precipitation in 2005 was again below average at 27.5 cm (Dixon, unpublished data). A summary of
vegetation response to tebuthiuron and grazing treatments
(Dixon, unpublished data) can be found in Smythe and
Haukos (2008) and Smythe (2006). In this paper, ‘‘migration’’ is defined as late February through the end of March,
and ‘‘breeding season’’ as April through August.
Total Bird Density
Density (individuals per hectare) of all species (n ¼ 28)
(Appendix) was not affected by the grazing treatment
(F[1,12] ¼ 0.20, p ¼ 0.66). Average total density was 40%
higher in tebuthiuron-treated plots than in untreated plots
(X treated ¼ 0.21 ± 0.01, X untreated ¼ 0.15 ± 0.01;
Restoration Ecology
Figure 1. Density of all avian species from February to July in
tebuthiuron-treated and untreated Sand shinnery oak communities
in Roosevelt County, New Mexico, in 2004 and 2005.
3
Response of Grassland Birds in Sand Shinnery Oak Communities
breeding areas for Cassin’s Sparrow, and although the occasional individual was observed during the winter, large
numbers were not present until the breeding season. Singing Cassin’s Sparrows were first observed in the study area
in April in 2004; however, they were first recorded a month
earlier, during March, in 2005. Average density (singing
males per hectare) of Cassin’s Sparrows on tebuthiurontreated plots (X ¼ 0.11 ± 0.009) was twice that on untreated
plots (X ¼ 0.05 ± 0.007; F[1,12] ¼ 19.31, p ¼ 0.0009; Fig. 2).
Average density was not affected by grazing (F[1,12] ¼ 2.42,
p ¼ 0.15). Average density of Cassin’s Sparrows was greater
in 2005 (X ¼ 0.10 ± 0.01) than in 2004 (X ¼ 0.06 ± 0.01),
but because of an interaction between year and month
(F[4,96] ¼ 2.99, p ¼ 0.02), the differences were observed only
in March, April, and May (Fig. 2). Regression analysis
across months revealed that in 2004, Cassin’s Sparrows
increased linearly in treated plots (F[1,36] ¼ 16.55, p ¼
0.0002) and remained constant in untreated plots (F[1,36] ¼
1.24, p ¼ 0.27). In 2005, the relationship was quadratic in
both the tebuthiuron-treated (F[1,48] ¼ 9.48, p ¼ 0.003) and
untreated plots (F[1,48] ¼ 11.79, p ¼ 0.001) (Fig. 2).
Meadowlark density (individuals per hectare) was not
affected by tebuthiuron treatment (F[1,12] ¼ 0.04, p ¼ 0.85)
or grazing (F[1,12] ¼ 0.88, p ¼ 0.37). On average, there was
a greater density of Meadowlarks in 2004 (X ¼ 0.06 ±
0.009) than 2005 (X ¼ 0.03 ± 0.004; Fig. 3); however, because of an interaction between year and month (F[5,132] ¼
5.51, p ¼ 0.0001), the difference was not consistent over
all months. Meadowlark density was similar in February
and July of both years but greater in 2004 for all other
months (Fig. 3). There was a linear relationship between
density and month in both 2004 and 2005 (F[1,60] ¼ 18.32,
p < 0.0001 and F[1,60] ¼ 22.60, p < 0.0001, respectively;
Fig. 3); this was predominantly influenced by the increase
in density from February to March (Fig. 3).
For a few species (Chihuahuan Raven [Corvus cryptoleucus], Grasshopper Sparrow [Ammodramus savannarum],
Loggerhead Shrike [Lanius ludovicianus], Mourning Dove
[Zenaida macroura], Vesper Sparrow [Pooecetes gramineus], Western Kingbird [Tyrannus verticalis], and WhiteCrowned Sparrow [Zonotrichia leucophrys]), we were able
to calculate density for only part of the sampling period.
These species were either migrants, not present in all
months, or residents present in low numbers (Smythe
2006).
Species Richness, Diversity, and Evenness
There was no overall tebuthiuron treatment (F[1,12] ¼
4.45, p ¼ 0.06) or grazing (F[1,12] ¼ 0.04, p ¼ 0.85) effect
on species richness. On average, species richness was
greater in 2005 than 2004 (X 2004 ¼ 6.46 ± 0.40, X 2005 ¼
8.14 ± 0.48), but because of an interaction between month
and year (F[5,120] ¼ 9.33, p < 0.0001), the difference between years was present only during late winter and spring
migration (February, March, and April) (Fig. 4).
Species diversity exhibited an interaction among tebuthiuron treatment, grazing treatment, and month (F[5,120] ¼
2.33, p ¼ 0.04) and between year and month (F[5,120] ¼ 5.92,
p < 0.0001). Thus, although diversity varied from month to
Figure 2. Density of Cassin’s Sparrows (Aimophila cassinii)
from March to July in tebuthiuron-treated and untreated Sand
shinnery oak communities in Roosevelt County, New Mexico,
in 2004 and 2005.
4
Figure 3. Density of Meadowlarks (Sturnella spp.) from February to
July in Sand shinnery oak communities in Roosevelt County, New
Mexico, in 2004 and 2005.
Restoration Ecology
Response of Grassland Birds in Sand Shinnery Oak Communities
Figure 4. Species richness, diversity, and evenness of avian species from February to July in Sand shinnery oak communities in Roosevelt
County, New Mexico, in 2004 and 2005. Uppercase letters indicate differences (p < 0.05) between years.
month, the variation was not consistent across grazing and
tebuthiuron treatments. Diversity was lower on ungrazed,
untreated plots than other treatment combinations in February and March of both years. On average, diversity was
greater in 2004 than 2005 (X 2004 ¼ 1.43 ± 0.07, X 2005 ¼
1.29 ± 0.09), but the difference between years was observed
only during February, March, and May (Fig. 4).
Evenness varied similarly to species richness in that
there was an interaction between month and year (F[5,132] ¼
15.70, p < 0.0001) and there were no effects from tebuthiuron (F[1,12] ¼ 0.38, p ¼ 0.55) or grazing (F[1,12] ¼ 0.16,
p ¼ 0.70) treatments. On average, evenness was closer to
Restoration Ecology
1 (where 1 represents a situation in which all species are
equally abundant) in 2004 than 2005 (X2004
¼ 0.78 ±
0.03, X 2005 ¼ 0.64 ± 0.04), but the difference was present
only during February, March, and May (Fig. 4). The decline in evenness in February and March of 2005 was
caused by large flocks of Chestnut-collared Longspurs
(Calcarius ornatus), and the decline in May was caused by
large flocks of Lark Buntings (Calamospiza melanocorys).
The influence of these flocks on species diversity is apparent: the dominance of these species caused the corresponding reduction in species diversity in the same
months of 2005.
5
Response of Grassland Birds in Sand Shinnery Oak Communities
Discussion
This study was part of a larger, long-term experiment in
which the goal was to restore the Sand shinnery oak community to a more historical grass/shrub balance and develop a grazing regime that would maintain that balance.
Our results reflect the initial response of grassland birds
to these restoration efforts, 4–5 years posttebuthiuron
application. Our results are limited to the most common
species in this community type and complicated by variable rainfall.
Under the climate conditions of this study, application
of tebuthiuron at 0.75 kg/ha to restore Sand shinnery oak
communities in Roosevelt County, New Mexico, resulted
in increased density of grassland birds. Increased density
on tebuthiuron-treated plots was present in both wet and
dry years. This differs from Martin (1990), who found no
difference in relative abundance of all species between
tebuthiuron-treated (using 0.5 kg/ha) and untreated shinnery oak communities in southeastern New Mexico. In
our study, migratory species appear to respond differently
to tebuthiuron treatment than resident species; the influx
of migratory Cassin’s Sparrows had the greatest influence
on the trends observed in density of all species. The grazing regime had very little impact on avian density. Month
and year were the other major factors affecting grassland
bird density; this temporal variation was likely in response
to habitat conditions that resulted from precipitation patterns. The quadratic relationship observed in the density
of all species stems largely from the patterns of migration and breeding in the study area. In addition to these
temporal patterns, abnormally abundant rainfall appears
to produce similar conditions to tebuthiuron treatment
(denser grass within the shrub component), which migratory species appear to key into more than resident species.
In this manner, the abundant rainfall of 2004 may have
masked treatment effects that might have appeared had
rainfall remained at or below average.
A migratory species in this study, Cassin’s Sparrow,
exhibited the greatest response to the treatments.
Although the rare individual was observed overwintering
on the study site, the conspicuous arrival of migratory
males was evident in April 2004 and March 2005, which is
when the density of all species on the treated and
untreated areas diverged. This indicates that density of
Cassin’s Sparrow was the greatest influence on overall
density trends on the study site. The linear density relationship in 2004 versus the quadratic relationship in 2005
suggests a habitat saturation effect: in 2004, untreated
areas were quickly saturated and remained at a constant
density, whereas density in the tebuthiuron-treated areas
expanded linearly. After the abundant rainfall in 2004,
vegetation in the study area was denser in the beginning of 2005 (Smythe 2006; Dixon, unpublished data). Untreated plots supported more birds in 2005 than 2004,
resulting in a quadratic relationship that mirrored that of
the tebuthiuron-treated plots. The peak density of Cas-
6
sin’s Sparrow in tebuthiuron-treated plots was similar in
2004 (0.31 singing males/ha) and 2005 (0.29 singing males/
ha), but occurred a month earlier in 2005, again suggesting
habitat saturation at a certain density. The maximum density of Cassin’s Sparrow in this study was comparable to
the density (0.27 singing males/ha) recorded by Schnase
(1984) in Mesquite mixed-grass prairie, but approximately
50% of the average density (0.57 singing males/ha) previously reported by Maurer (1985) in grasslands.
Meadowlarks, the resident species for which we had
consistent data, exhibited little response to either treatment. The neutral response of Meadowlarks to tebuthiuron
treatment was consistent with Martin’s (1990) results for
Meadowlarks in treated and untreated shinnery oak areas.
Gruver and Guthery (1986) also found Meadowlark densities to be equal on treated and untreated areas of Mesquite
(Prosopis spp.) 13 years after Mesquite control, and
Castrale (1982) recorded no population response to sagebrush control by Western Meadowlarks. The change in density from February to March in both years likely represents
an influx of migrating Meadowlarks into the study area for
the breeding season, but more information on Meadowlark
movements is needed to support this assertion.
Year and month also influenced density of all species
because manifestations of migratory patterns or improved
habitat conditions from increased precipitation. The increase in density from 2004 to 2005 stems from increases in
migratory species, predominantly Grasshopper Sparrow
(which were not present during migration in 2004; Smythe
2006) and also from the earlier arrival of Cassin’s Sparrow.
The increase in density during the breeding season might
be related to the above-average rainfall in 2004. This type
of response by Cassin’s Sparrow to precipitation was hypothesized by Hubbard (1977). Similarly, Igl and Johnson
(1995, 1999) reported the change in status of Le Conte’s
Sparrow (Ammodramus leconteii) from an uncommon
breeding species to the most abundant over a period of 6
years that coincided with the amelioration of drought conditions and associated changes in habitat in the northern
Great Plains. Ruth (2000) summarized anecdotal evidence
suggesting similar distribution changes in response to precipitation by Cassin’s Sparrows. After the peak migration
period, density of all species was similar between years,
indicating that the arrival of migratory breeding sparrows
had the greatest influence on overall density trends.
Grasslands generally have low densities of birds, but
densities on this study site were considerably lower
than those reported in several other grassland studies (e.g.,
Giezentanner 1970; Wiens 1973; Cody 1985; Igl & Ballard
1999; DeJong 2001). Current low densities in Sand shinnery
communities might indicate an ecological sink or reduced
habitat carrying capacity, although the normal productivity
for these communities is not well documented.
Avian species richness, evenness, and diversity were only
minimally affected by the tebuthiuron and grazing treatments. This partially agrees with Lueders et al. (2006), who
found that neither species richness nor bird densities were
Restoration Ecology
Response of Grassland Birds in Sand Shinnery Oak Communities
higher in structurally more diverse habitat in North
Dakota. It appears that tebuthiuron treatment at this application rate and resulting vegetation response affect mainly
the numbers of birds present, not which species or in what
proportions they occur. Tebuthiuron-treated plots supported a greater density of the same community of birds as
untreated plots (Smythe 2006), although additional longterm monitoring, spanning wet–dry cycles and maturing
vegetation communities, will be necessary to determine the
full community structure response to treatments.
The moderate grazing regime (Holechek et al. 2001)
implemented in this study appears to have little effect on
grassland bird populations in this region, likely because
the mosaic of grazed and ungrazed plots offered structural
diversity. It is also important to emphasize that grazing
was deferred on the study plots for 3 years after the tebuthiuron treatment, longer that the 1- to 2-year deferrals
recommended by most current management guidelines
(Peterson & Boyd 1998). Wiens and Dyer (1975) suggested that because grassland birds are so ecologically
plastic, avifaunal shifts occur only after some threshold of
habitat change has been passed, and therefore, many species of grassland birds would be unresponsive to moderate
levels of grazing, such as occurred in this study. These
results indicate that short-duration grazing regimes, based
on the correct stocking rate and knowledge of available
forage, are not detrimental to grassland bird populations.
Bourliere and Hadley (1970) and Wiens (1973) suggested
that migratory grassland birds live and reproduce off of the
excesses produced by grasslands. Large seasonal or annual
fluctuations in weather and production may produce surplus organic and nutrient matter that can be exploited by
migrant species. Cody (1985) suggested that in response to
climatic and production variations, grassland birds maintain a generalized morphology and behavior and practice
resource tracking. This phenotypic plasticity in behavior
and diet allows grassland birds to select geographic areas
from year to year based on available resources. Igl and
Johnson (1999) described how erratic population fluctuations are characteristic for most grassland passerines
because they are highly dependent on specific vegetation
features that vary dramatically in response to local moisture
conditions. Given that seed and herbaceous production was
considerably higher on treated plots than on untreated
plots (Dixon, unpublished data), this resource tracking may
explain the differences in density patterns between treated
and untreated plots. This also might explain why migrant
sparrows responded differently to shinnery oak control
than did the year-round resident Meadowlarks. Meadowlarks are exclusively insectivorous (Wiens 1973) and
Cassin’s Sparrow largely so during the breeding season, but
Cassin’s Sparrow also consume seeds and plant matter
(Ruth 2000). Our results would likely be more pronounced
in average or below-average precipitation years.
In this study, Sand shinnery oak plots treated with tebuthiuron to restore a more historical grass/shrubland combined with a moderate grazing system supported a greater
Restoration Ecology
density of Cassin’s Sparrow (a migratory species) than
untreated plots and equal densities of Meadowlarks (a resident species) as untreated plots. This suggests that restoring
Sand shinnery oak communities to a more historical grass/
shrub balance can create vegetation heterogeneity that benefits migratory grassland birds and does not appear to harm
residents. A carefully managed grazing regime also does
not appear to negatively impact grassland birds; however,
grazing must be managed to maintain restoration efforts,
and continued monitoring is needed to assess the long-term
effects of restoration. There are approximately 1,068,370
hectares of shinnery oak in New Mexico (Garrison &
McDaniel 1982) and about 2 million hectares of Sand shinnery communities across the southern Great Plains (Peterson & Boyd 1998). In addition, these Sand shinnery
communities are important in winter to breeding birds of
other prairie communities (Rich et al. 2004). Although our
results may not apply directly to most of the priority species
in these communities, they may provide general guidance
on how species may respond to these management practices. The exception to this was Cassin’s Sparrow, a species
of continental importance (Rich et al. 2004), which may
benefit greatly from the management described in this
study. If drought conditions continue to be the norm in this
region, tebuthiuron treatment may help provide the optimal grass/shrub mix for an increased density of grassland
birds that would otherwise be dependent on normal or
above-normal precipitation. Finally, vegetation variables
influencing grassland passerine density can differ from
those affecting nesting success (Winter et al. 2005), and
thus avian density responses to these treatments do not
imply similar effects on nesting success. Nest productivity
and reproductive success are other important factors to
consider when evaluating habitat quality and the impacts of
management practices in Sand shinnery oak communities.
Implications for Practice
Sand shinnery oak plots restored to a grass/shrub mix
with low doses of tebuthiuron (0.73 kg/ha) supported
a greater density of spring migrants and breeding
birds than untreated plots.
d Tebuthiuron treatment may create a grass/shrub mix
normally restricted to years of above-average precipitation. This could increase densities of some migratory grassland bird species such as Cassin’s Sparrow
and does not appear to harm resident species such as
Meadowlarks. Further research, spanning longer
wet–dry cycles and maturing plant communities, will
be necessary to completely determine avian response
to these restoration efforts.
d A carefully managed, moderate grazing regime also
does not appear to negatively impact grassland bird
density; however, grazing must be managed to maintain
restoration efforts, and continued monitoring is needed
to determine the long-term effects of restoration.
d
7
Response of Grassland Birds in Sand Shinnery Oak Communities
Acknowledgments
We thank D. B. Wester for assistance with statistical analyses and C. L. Dixon of Wildlife Plus Consulting for sharing vegetation and rainfall data. We also thank A. Andrei
and J. Hull for their conscientious data collection and hard
work in the field. This study was funded by a grant from
the National Fish and Wildlife Foundation, with support
from the Region 2 Migratory Bird Office of the U.S. Fish
and Wildlife Service; the Department of Natural Resources Management, Texas Tech University; and Grasslans
Charitable Foundation. This is manuscript T-9-1146 of the
College of Agricultural Sciences and Natural Resources,
Texas Tech University.
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Appendix. Summary and status of 28 avian species detected on the study site in Roosevelt County, eastern New Mexico.
Common Name
Scientific Name
American Kestrel
Ash-throated flycatcher
Blue Grosbeak
Brewer’s Sparrow
Bullock’s Oriole
Burrowing Owl
Cassin’s Sparrow
Chestnut-collared Longspur
Chihuahuan Raven
Chipping Sparrow
Eastern Meadowlark
Field Sparrow
Grasshopper Sparrow
Lark Bunting
Lark Sparrow
Loggerhead Shrike
Mourning Dove
Northern Bobwhite
Northern Mockingbird
Prairie Falcon
Savanna Sparrow
Sage Thrasher
Scissor-tailed Flycatcher
Swainson’s Hawk
Western Kingbird
Western Meadowlark
White-crowned Sparrow
Vesper Sparrow
Falco sparverius
Myiarchus cinerascens
Guiraca caerulea
Spizella breweri
Icterus bullockii
Athene cunicularia
Aimophila cassinii
Calcarius ornatus
Corvus cryptoleucus
Spizella passerina
Sturnella magna
Spizella pusilla
Ammodramus savannarum
Calamospiza melanocorys
Chondestes grammacus
Lanius ludovicianus
Zenaida macroura
Colinus virginianus
Mimus polyglottos
F. mexicanus
Passerculus sandwichensis
Oreoscoptes montanus
Tyrannus forficatus
Buteo swainsoni
T. verticalis
Sturnella neglecta
Zonotrichia leucophrys
Pooecetes gramineus
Species Nests
in Sand
Shinnery Oak
Communities
Species-Specific
Density
Estimated
in This Study
Species of
Continental
Importance in
Southwest Regiona
Priority
Species in
New Mexicob
Yes
Yes
Yes
No
Yes
Yes
Yes
No
Yes
No
Yes
No
No
No
Yes
Yes
Yes
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
No
No
No
No
No
No
No
No
Yes
No
Yesd
No
Yesc
No
Yesd
No
No
Yesd
Yesd
No
No
No
No
No
No
No
Yesd
Yesc
Yesd
Yesd
No
No
No
Yes
No
No
Yes
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes
No
No
No
No
No
No
No
Yes
No
No
No
Yes
a
As defined by the Partners in Flight North American Landbird Conservation Plan (Rich et al. 2004).
As defined by the New Mexico Partners in Flight Bird Conservation Plan (NMPIF 2007).
c
Species grouped for density estimate.
d
Results presented in Smythe (2006).
b
Restoration Ecology
9