This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Bird Habitat Relationships in Desert Grasslands
Michael L. Morrison, Adjunct Professor, Department of Biological
Sciences, California State University, Sacramento, CA
G
rassland-associated bird species have experienced declines greater than any
other group of birds in the United States, including neotropical bird species
of forested habitats. Numerous studies have focused on grassland bird conservation but primarily in plains grasslands of the Midwest. Less attention has focused
on bird species associated with desert grasslands of the southwestern United States.
Bird species known to occur in this region have also experienced similar long-term
declines.
The focus of this study is to examine how bird species and assemblages distribute themselves across a vegetation gradient in desert grasslands of southeast
Arizona and southwest New Mexico. We also wish to determine the range of
vegetation types that particular bird species select and if habitat preference differs
between seasons. This study was conducted in the San Simon and San Bernardino
Valleys of Cochise County, Arizona, and the Animas Valley of Hidalgo County,
New Mexico. Our specific study questions were:
1. What characteristics of the vegetation influence the distribution and a bundance of particular bird species?
2. Where, along a gradient of decreasing woody cover and increasing grass
cover, does a shift in bird assemblages occur?
3. How do characteristics of the vegetation in desert grasslands affect distributional patterns of bird assemblages?
4. How do characteristics of the bird community along our gradient differ
between summer and winter seasons?
During the autumn of 1996, we placed 28 1-km transects across a range of
vegetation types that represented the diversity of desert grassland within the region. Twenty-two of the transects were placed in Cochise County, Arizona, and
six in Hidalgo County, New Mexico. Each transect represented a study site. Our
goal in selecting sites was to sample as much of the range in grassland conditions
as existed within our study region. All transects were at least 500 m apart to avoid
interdependence of samples, and all transects were at least 200 m from riparian
corridors to avoid misinterpretation of bird community-vegetation relationships.
We attempted to position each transect in generally uniform areas to permit birdvegetation associations in the larger area surrounding the transect.
During the spring of 1997, two of our 28 sites were burned unexpectedly. We
replaced these two sites at the beginning of the summer sampling period. These
sites were not included in our between-season comparisons of the bird community.
Bird surveys were conducted from November to February 1996-1997, and
May to August 1997. These sampling periods represented winter and summer
breeding seasons respectively. Surveys were conducted using the variable-widthstrip transect method. Each transect was visited four to five times within each
season.
122
USDA Forest Service Proceedings RMRS-P-10. 1999.
Bird and Vegetation Association
Morrison
During the fall of 1996 we sampled vegetation along the 28 transects to
determine foliage structure and plant species composition. We randomly placed
ten 40-m transects perpendicular to the 1-km survey transects. 1\1easurements
were taken at 1-m intervals along each 40-m transect. Using the line-intercept
method, we measured horizontal cover as a percent by class. Mean percent cover
by class and mean vertical height diversity at each study site was calculated as the
estimate obtained for each 40-m transect divided by the total number of transects
sampled (n=lO). To obtain a measure of vertical height diversity, we recorded the
height class at which vegetation intercepted a vertical line. We detected a total of
49 bird species during November to February 1996-1997 and May to August
1997. Of these species, 22 (45%) were migrants.
A total of 33 bird species were detected during winter. Of these, 13 ( 39.4%)
are known to be wintering migrants to the region. Sparrow species made up 53.8%
(7 out of 13) of the migrants. At more than 25% of our sites we detected more
than five sparrow species (range= 0-10). Grasshopper sparrows, eastern meadowlarks, horned larks, and vesper sparrows were the most commonly occurring species among study sites. Grasshopper sparrows and horned larks were detected only
at sites with < ll% woody cover and reached their highest mean frequencies at sites
with no woody cover. We detected eastern meadowlarks at all sites.
A total of 36 bird species were detected during summer. Of these species, ll
(30.6%) are known to be summer migrants to the region. Eastern meadowlarks,
lark buntings, scaled quail, black-throated sparrows, and mourning doves were
the n1ost commonly occurring species among study sties. Horned larks and grasshopper sparrows were confined to those sites with< 11.0% woody cover and reached
their highest densities at sites with no woody cover. We detected eastern meadowlarks at all sites.
Breeding was observed for 19 bird species during the summer breeding season. All species included in our analyses were confirmed breeding except for Botten's
sparrow and ash-throated flycatchers. Those species not included in our analyses
but confirmed as breeding included lark sparrows, rufous-crowned sparrows, and
verdins. Black-throated sparrows were confirmed breeding across the widest range
of woody cover levels. Horned larks exhibited the narrowest range of breeding
areas relative to woody cover.
Results of this study, along with earlier work in desert grasslands of Arizona
and New Mexico, show that woody plant cover strongly influences the presence
and relative abundance of particular bird species. We focused on a range of sites
that did not include high densities of tall (>1.5-m) tree species such as mesquite
and acacia. Woody plants on our study areas consisted primarily of shrubs, subshrubs, and small trees ( <1.5-m) and did not exceed levels >15% except at one
site. This feature was demonstrated by the low occurrence of species such as thrashers, verdins, black-tailed gnatcatchers, kingbirds, and other birds associated with
the presence of tall, dense tree species such as mesquite.
We conclude that a gradient of increasing woody cover and shrub species
richness along with decreasing grass cover is the primary gradient of our study to
which the bird species responded to on a regional scale. Our conclusion is supported by the high number of correlations between bird species and woody plant
related variables revealed for both the winter and summer seasons.
Bird species occurring within our study region were similar to those found in
studies conducted in the Sonoita and San Rafael Valleys, >90 km to the west.
Thus, our study area apparently provides habitat for a number of desert grassland
associated species. The region clearly contains vegetation types that range from
pure native grasslands (semidesert/plains grassland) and grasslands intermixed
USDA Forest Service Proceedings RMRS-P-10. 1999.
123
Morrison
Bird and Vegetation Assocation
with shrubs, sub-shrubs, and small trees. Notable properties of the bird communities we studied were the rarity of species associated with high levels of woody
cover, the presence of sensitive grassland birds, and a relatively rich array of wintering migrant sparrow species. Our results suggest that woody cover plays an
important role in determining the presence and abundance of bird species within
our study region. Thus, a gradient of woody cover may reveal recognizable shifts
in bird assemblages at some level. Levels of woody cover <10% did appear to
support sensitive grassland species such as grasshopper sparrows and chestnutcollared longspurs, and levels >10% support a different and richer set of species.
During summer we confirmed breeding status for grasshopper sparrows and horned
larks. The majority of confirmations were at sites with low levels of woody plant
cover ( <4%). Thus, within this region there were areas where adequate habitat
exists for these sensitive grassland species.
Our results show numerous bird species such as grasshopper sparrows, rufous-crowned sparrows, horned larks, and scaled quail inhabit this region and
reproduce. How much these bird populations contribute to desert grasslands as a
whole in southeast Arizona is unknown but may be substantial. Some species characteristic of desert grasslands such as rufous-winged sparrows and Sprague's pipit
were detected rarely or not at all. This fact is either an inability of our sites to
represent the full range of grassland types in the region or merely the rarity of the
species. Regardless of the reason, our study has provided valuable information on
the grassland bird communities inhabiting the region where Arizona, New Mexico,
and Mexico meet. This study provides valuable information on desert grassland
bird communities inhabiting a region that has received little attention. Because of
the influence this region may have on trends in desert grassland bird populations,
we recommend that long-term surveys are initiated to assess the role this region
may have in future conservation of sensitive desert grassland bird species.
124
USDA Forest Service Proceedings RMRS-P-10. 1999.