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2016 SOUTHEASTERN NATURALIST 15(3):382–393
Habitat Selection of Wild Pigs and Northern Bobwhites in
Shrub-dominated Rangeland
Susan M. Cooper1,* and Shane S. Sieckenius1
Abstract - Sus scrofa L. (Wild Pig) are known to eat the eggs of ground-nesting birds, but
it is unknown to what extent they have an impact on populations of Colinus virginianus L.
(Northern Bobwhite). We combined data from 2 prior studies conducted on a large ranch
in South Texas to assess comparative habit use and selection by Wild Pigs and Bobwhites.
Both species were distributed throughout the ranch, but their preferred habitats were very
different. Bobwhites favored upland habitats, particularly areas with deep, sandy soils,
while Wild Pigs favored low-lying habitats with clay soils. We conclude that depredation
of Bobwhite nests by these invasive mammals may be restricted by the thermoregulatory
requirement for Wild Pigs to stay near riparian areas. The extent of this effect will depend
on the availability and spatial arrangement of water and riparian areas on the landscape.
Introduction
Colinus virginianus L. (Northern Bobwhite, hereafter Bobwhite) are a valuable
hunting and recreational wildlife resource that provides income to rural economies
(Burger et al. 1999, Conner 2007). Consequently, the long-term population decline
of this species throughout most of its range (Brennan 1991, Hernández et al. 2013)
is of concern to many sportsmen, conservationists, and landowners. In Texas, the
rate of decline of Bobwhite populations averages 2.78% a year (Sauer et al. 2014).
Within the South Texas Plains ecoregion, numbers of Bobwhites have been consistently
low since the mid-1990s and show little sign of recovery despite much effort
in managing land for this species (TPWD 2014).
In contrast, Sus scrofa L. (Wild Pig) are expanding in range and abundance in the
US (Gipson et al. 1998). In Texas, the Wild Pig population, derived from feral Sus
scrofa domesticus) Erxleben (Domestic Pig) often hybridized with Eurasian Wild
Boar; Wilson and Reeder 2005), was estimated to be around 1 million animals in
the 1990s (Taylor 1991, 1993). However, a recent synthesis of current literature and
harvest records by Timmons et al. (2012) estimated that there are now 2.6 million
(range = 1.8–3.4) Wild Pigs in Texas. Furthermore, based on their high fecundity,
simulation modeling predicts the potential for even larger populations in the future
(Mellish et al. 2014).
There is concern that the ever-growing population of Wild Pigs may hinder recovery
of Bobwhite populations. Wild Pigs readily consumed eggs in nest predation
studies that used artificial nests baited with Gallus gallus domesticus L. (Domestic
Chicken) or hatchery-produced Bobwhite eggs (Cooper 2006, Tolleson et al.
1993), and they have been documented depredating nests of Bobwhites (Rollins
1Texas A&M AgriLife Research, 1619 Garner Field Road, Uvalde, TX 78801. *Corresponding
author - s-cooper@tamu.edu.
Manuscript Editor: John C. Kilgo
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and Carroll 2001) and Meleagris gallopavo L. (Eastern Wild Turkey; Dreibelis
et al. 2008). It is possible that nest predation by Wild Pigs could have a negative
effect on local population-recruitment of Bobwhites, particularly if there is considerable
overlap in their habitat use during Bobwhite nesting season. Depredation
by non-native Wild Pigs has been implicated in declines of several populations of
ground-nesting birds on islands (Donlan et al. 2007, Miller and Mullette 1985) and
in Europe (Nyenhuis 1991). In Texas, Petty et al. (2007) found that control of Wild
Pig populations has been effective in aiding restoration of Eastern Wild Turkey. Yet
to date, quantitative evidence of the impact of Wild Pig depredation on Bobwhite
populations is lacking (Rollins et al. 2007, West et al. 2009).
In this study, we synthesized information from 2 independent studies that we had
previously undertaken. One study assessed the distribution and movements of GPScollared
Wild Pigs on a large ranch in Texas (Cooper et al. 2010) in order to model
potential disease-transmission between Wild Pigs and Bos taurus L. (Domestic
Cattle). The other study is an ongoing, long-term monitoring program on Bobwhite
populations on the same ranch and other member ranches of the South Texas Covey
Connection (Cooper et al. 2009). We combined data from these 2 studies to identify
habitats within South Texas rangeland in which Wild Pigs are most likely to overlap
in distribution with nesting Bobwhites. Through depredation of nests, mammals are
the main predators of quail during the breeding season (Rollins and Carroll 2001);
thus, our hypothesis was that knowledge of the comparative distribution of Wild
Pigs and Bobwhite during Bobwhite breeding season should be important for assessing
the negative effects of Wild Pigs on Bobwhite populations.
Field-Site Description
The main study area was a 30,521-ha low-fenced ranch in Zavala County, TX,
within the South Texas Plains ecoregion. Supplemental data on Bobwhite populations,
but not Wild Pigs, over the same time period was also available from 3
additional ranches in Zavala and Frio counties, which together covered 10,950 ha
of habitat similar to that on the main study-ranch.
The regional climate is subtropical and semi-arid with hot, dry summers (mean
temperature = 37 °C in July) and mild winters (mean temperature = 4 °C in January)
(Griffith et al. 2007). The 100-y average for annual precipitation is 555 mm (NOAA
2014), but there is great annual variability. During the 11 y of Bobwhite population
surveys from 2004 to 2014, annual precipitation measured on-site averaged 523
mm, but ranged from 293 to 870 mm/y. During the 2 y of the Wild Pig study, from
2004 to 2006, rainfall was slightly above average at 635 mm/y.
The South Texas Plains ecoregion is characterized by gently rolling topography
vegetated by a diverse, thorn-shrub community (Griffith et al. 2007). Soil types on
the study ranch were 14% clay, 66% clay-loam, 16% sandy-loam, and 3% deep,
sandy soil (USDA-NRCS 2012). The 3 additional ranches lacked deep sandy soils.
Soil types were 5% clay, 34% clay-loam, 12 % loamy-sand, 33% sandy-loam, and
15% gravelly-loam. The most characteristic woody plant on clay and clay-loam
soils was Prosopis glandulosa Torr. (Honey Mesquite), growing in combination
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with Opuntia engelmannii Salm-Dyck ex Engelm. (Pricklypear Cactus) and a diverse
array of drought-tolerant shrubs, such as Guaiacum angustifolium Engelm.
(Guayacan), Zizyphus obtusifolia (Hook. ex Torr. & A. Gray) Gray (Lotebush),
Castela erecta Turpin (All-thorn), and Celtis pallida Torr. (Spiny Hackberry).
Mesquite was less abundant on the sandy-loam soils, which were densely vegetated
with shrubs of Vachellia rigidula (Benth.) Seigler & Ebinger (Blackbrush Acacia),
Senegalia berlandieri Brittom & Rose (Guajillo), and Leucophyllum frutescens
(Berl.) I.M. Johnst (Cenizo). Some of the shrubland had been cleared in alternating
strips to increase grazing for cattle and wildlife. Native grasses were abundant on
the ranch but the introduced pasture grass Cenchrus ciliaris L. (Buffelgrass) was
dense on the clay-loam soils. The area of sandy soil on the northern section of the
ranch was characterized by more-open grassland with Mesquite, Pricklypear, and
mottes (groves) of Sideroxylon lanuginosum Michx. (Spiny Bumelia). Grasses on
the sandy soils were all native species characterized by the presence of Schizachyrium
scoparium (Michx.) Nash (Little Bluestem) (Correll and Johnson 1970, ITIS
2015). Many ephemeral drainages leading into the Turkey Creek system, parts
of which usually held water year-round, were present on the landscape. Riparian
areas were predominantly narrow bands of dense shrubs and Mesquite trees along
the drainages. Riparian areas around permanent water bodies contained taller trees
including Quercus virginiana Mill. (Live Oak) and Carya illinoinensis (Wangenh.)
K. Koch. (Pecan). All pastures contained earthen stock-ponds to provide water for
Cattle. There were 5 cultivated and irrigated fields of 71–88 ha planted with Sorghum
x almum Parodi (Columbus Grass) and 14 scattered food-plots of 0.81–2.02
ha (2–5 ac) planted with Avena sativa L. (Oats) for wildlife.
The ranch was used for game production and cattle grazing. Hunted game species
included Odocoileus virginianus Zimmermann (White-tailed Deer), Wild Pigs,
Pecari tajacu L. (Javelina), Rio Grande Wild Turkey, Bobwhite, and Callipepla
squamata Vigors (Scaled Quail). We did not measure Wild Pig density on the study
site, but Timmons et al. (2012) estimated that Wild Pigs in Texas occur at a density
of 3.4–6.3 /km2.
Methods
Assessment of Bobwhite abundance and distribution
From mid-April through May 2004−2014, we conducted surveys of Bobwhites
by recording the abundance of calling roosters at 10 call-stations spaced equidistantly
along a 16-km route on each ranch. Following standard procedures for spring
call-counts (Guthery 1986, Hansen and Guthery 2001, Rollins et al. 2005), we monitored
each station for 5 min on 3 non-consecutive mornings. At each site, we noted
the number of Bobwhites calling and we recorded their locations by compass direction
and estimated distance of each bird from the observer. The suggested distance
over which Bobwhite calls are audible is ~600 m (Rollins et al. 2005); however,
during this study, we found that a distance of 400 m (survey area 50 ha) was a more
appropriate estimate because our ability to hear Bobwhites in this shrubby country
was reduced by the background noise-level of birdsong, including numerous Mimus
polyglottos L. (Northern Mockingbird) partially imitating Bobwhite calls.
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We used ArcView 10 (ESRI, Redlands, CA) to plot the 400-m-radius circles
around each survey point and Bobwhite locations on digital aerial photography from
TNRIS (2014). We overlaid a map of ecological sites derived from the USDA-NRCS
web soil-survey (2012) to determine proportions of each ecological site (habitat
type) within the survey circles. In this paper, we capitalize ecological site-names to
differentiate those names from similarly named soil types. Ecological sites are derived
from a combination of soil type and topography that influences the vegetation
communities at each site (Table 1); thus, they delineate land units that share similar
characteristics over a wide geographic area (USDA-NRCS 2011).
We classified an ecological site as selected or avoided if the proportional occurrence
of calling Bobwhites differed significantly from the proportion of that
ecological site available within the combined counting circles on the ranch. We tested
differences between proportional use and availability of ecological sites with the chisquare
(χ2) statistic and accepted differences at P < 0.05 (Neu et al. 1974). Preference
for individual sites was confirmed by individual chi-square tests with Bonferroni adjustment
(Byers et al. 1984, McDonald 2014). Given that home ranges of Bobwhites
in the breeding season are only10–15 ha in extent (Haines et al. 2004, Liu et al. 2002),
we made the assumption that Bobwhites were likely to nest close enough to their calling
sites to remain within the same ecological site as the calling rooster. The average
number of calling roosters/site is presented with the standard error (SE). We did not
describe the distribution of Bobwhites relative to landscape features, such as riparian
areas and ranch roads, because standard protocol for Bobwhite survey points places
them near access roads and outside riparian zones.
Table 1. Proportions of ecological sites within the study ranch in the South Texas Plains ecoregion.
A more-detailed description of each ecological site can be found at USDA-NRCS (2011) Ecological
Site Information System.
Ecological site Soil characteristics % occurrence
Upland areas
Clay Flat Clay 6.49
Saline Clay Clay, saline 6.78
Clay Loam Clay-loam, on caliche 46.97
Rolling Hardland Clay-loam, on shale, saline 11.27
Loamy Sand Loamy-sand 0.04
Sandy Loam Sandy-loam 5.50
Gray Sandy Loam Sandy-loam 8.56
Shallow Sandy Loam Sandy-loam on sandstone 0.41
Sandy Deep sand 3.15
Shallow Ridge Gravelly-loam, on caliche 0.26
Gravelly Ridge Gravelly-loam 0.30
Lowland and drainages
Clayey Bottomland Deep clay 0.67
Claypan Prairie Fine sandy-clay-loam 7.07
Tight Sandy Loam Sandy-loam 1.77
Loamy Bottomland Silty-clay-loam 0.20
Lakebed Clay-loam 0.02
Water Stock ponds 0.54
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Habitat use and selection by Wild Pigs
Information on habitat use and selection by Wild Pigs is based on GPS collar
(BlueSky Telemetry® Limited, Aberfeldy, Scotland; and Lotek Wireless Incorporated,
Newmarket, ON, Canada) data collected on the main study ranch from July 2004
to August 2006 (Cooper et al. 2010). Collars provided locations of the animals every
15 min at spatial accuracy of ± 5 m (Hulbert and French 2001, Moen et al. 1997). We
used data from the 2 spring (March–May) trials and 2 summer (June–August) trials
to describe the distribution of 16 Wild Pigs during the Bobwhite breeding season,
which lasts from April to September (Hernández and Peterson 2007). We used box
traps to catch the Wild Pigs for collaring. We ear-tagged captured Wild Pigs to prevent
re-use of previously collared animals, and we were careful not to collar more
than 1 animal from each sounder, or family group. Trapping and handling of Wild
Pigs followed Animal Use Protocols #2002-380 and #2005-281 approved by Texas
A&M University Institutional Animal Care and Use Committee.
In addition to ecological sites, we assessed Wild Pig distribution relative to landscape
features digitized from digital aerial photography from TNRIS (2014). These
features included: (a) creeks and drainages buffered at 50 m to delineate riparian
vegetation, (b) water (stock ponds) buffered at 50 m due to fluctuating water levels,
(c) cultivated fields which were fenced and therefore not buffered, and (d) ranch
roads buffered at 20 m from the center-line. Areas of these landscape features were:
riparian = 3096 ha, water = 636 ha, cultivated = 896 ha, and roads = 3218 ha. We
classified the remaining 25,033 ha (82%) of rangeland that were not within 1 of
these 4 categories as shrubland. Supplemental-feeding sites for deer were fenced to
exclude Wild Pigs and Cattle; thus we did not include them in the analysis.
We determined use of ecological sites or landscape features by Wild Pigs each
season by the proportion of GPS locations falling within each ecological site or
feature compared to the proportional extent of that ecological site or feature within
the study area. We employed individual chi-square (χ2) tests with Bonferroni adjustment
to verify site selection (Byers et al. 1984, McDonald 2014). For Wild Pigs,
we designated as available habitat the 25,888 ha of the ranch south of Texas State
Highway 57 because none of the home ranges of the collared animals extended
across the highway.
Results
Habitat use and selection by Bobwhites
On the study ranch, habitat selection by Bobwhites was dominated by a strong
preference for the Sandy ecological site (Table 2) and a corresponding avoidance of
the abundant Clay Loam sites (χ2
3 = 24.11, P < 0.001). Occurrence of Clay Flat sites
within the survey circles was low, but a trend towards avoidance of this ecological
site was evident. Over the 11 years, the mean ± SE number of roosters heard was 5.0
± 1.0 roosters/50-ha survey circle in Sandy sites, compared to 2.1 ± 0.5 roosters/50
ha in Clay Loam sites, and 1.8 ± 0.4 roosters/50 ha in Clay Flat sites.
On the 3 additional ranches, which did not have areas of deep sandy soils but
had various loamy soils, no clear preference for any specific ecological sites by
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2016 Vol. 15, No. 3
Bobwhites was evident (χ2
7 = 9.80, P > 0.05; χ2
5 = 2.02, P > 0.05; and χ2
7 = 6.44, P >
0.05), although there was a slight but not statistically significant tendency for fewer
Bobwhites to be heard in the low-lying habitats (Fig. 1).
Table 2. Ecological site preference of Northern Bobwhites within the study ranch in the South Texas
Plains ecoregion, April 2004–2014. * indicates selected, † indicates avoided.
% of calling area
Ecological sites within ten 50-ha survey circles % of Bobwhites heard χ2 value
Clay Flat 12.78 9.20 1.00
Clay Loam 45.35 26.31 7.99†
Sandy Loam 2.02 0.86 0.67
Sandy 39.69 63.64 14.45*
sum χ2 = 24.11
df = 3
P < 0.001
Figure 1. Distribution of Northern Bobwhites on ecological sites on 4 ranches in the South
Texas Plains during spring 2004−2013. Study ranch in black, additional ranches in grey:
ranch H (dark grey), ranch W (light grey), ranch S (mid-level grey).
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Distribution and habitat selection of Wild Pigs
The Wild Pigs fitted with GPS collars included 1 adult boar, 8 subadult males,
and 7 adult and subadult females. We did not sedate the animals for collaring, and
thus could not weigh them, but we determined that shoulder height of these animals
ranged from 84 cm to 61 cm (mean = 64 ± 8 cm).
Wild Pigs were more often located in habitats with clay soils than in those with
clay-loam or sandy-loam soils (χ2
2 = 43.13, P < 0.001). We could not assess use
of sandy soils because none of the GPS-collared Wild Pigs crossed the highway to
access the 961-ha Sandy ecological site favored by Bobwhites. However, we rarely
saw signs of rooting in this area, suggesting that Wild Pigs were not abundant
within this area with sandy soil. The distribution of Wild Pigs was consistently
associated with selection for proximity to water sources (χ2
8= 91.71, P < 0.001).
The ecological sites preferred by Wild Pigs were Clay Flat, and to a lesser degree,
Claypan Prairie. Non-preferred sites included Clay Loam and to a lesser extent
Rolling Hardland, Grey Sandy Loam, and Sandy Loam sites (Table 3).
There was some variation in the distribution of Wild Pigs during the early
(spring) compared to the late (summer) breeding season of Bobwhites. During
spring, preference of Wild Pigs for Clay Flats was not evident. Instead, Wild Pigs
extensively used weedy, fallow fields planted within Clay Loam ecological sites,
and thus, showed a preference for this habitat type. During summer, Wild Pigs selected
low-lying Clay Flat and Claypan Prairie ecological sites and tended to avoid
upland areas including Clay Loam, and to a lesser extent Rolling Hardland, Grey
Sandy Loam, and Sandy Loam sites.
Landscape features also influenced the distribution of Wild Pigs (Table 4). They
were strongly attracted to water sources and riparian zones, but neither selected nor
avoided ranch roads. Although shrubland covered 82% of the ranch, only 1/3 of
Wild Pig locations were in this type, indicating avoidance of these extensive areas
(χ2
1 = 58.13, P < 0.001). Cultivated fields constituted less than 3% of the study area. In
spring, Wild Pigs heavily used these fallow grain-fields, particularly at night (75.9
Table 3. Seasonal habitat use and selection by Wild Pigs in the South Texas Plains ecoregion during
spring (n = 10), and summer (n = 6), 2004–2006. * indicates selected, † indicates avoided.
% of Spring Summer
Ecological site area % χ2 % χ2
Clay Flat 6.13 8.62 1.01 29.20 86.54*
Saline Clay 7.91 3.51 2.45 13.60 4.08
Claypan Prairie 7.78 8.79 0.13 22.50 27.62*
Rolling Hardland 13.10 6.23 3.63 4.25 6.01
Clay Loam 47.20 67.4 8.60* 15.90 20.72†
Gray Sandy Loam 8.66 1.20 6.43† 0.34
Sandy Loam 4.84 0.29 4.28† 0.13 4.58
Water 0.59 3.78 17.27* 7.52 81.50*
sum χ2 43.80 231.39
df 7 7
P less than 0.001 less than 0.001
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2016 Vol. 15, No. 3
± 23.7% of locations). Wild Pigs tended to spend slightly less time (10.6%) in riparian
areas at night than during the day, but variability between individuals was high,
and overall differences in distribution in riparian areas and ecological sites between
day and night were not significant.
Discussion
Combining data from separate studies for purposes beyond their original objectives
can provide useful if not perfect information at a fraction of the time
and cost of initiating new studies. Our investigation, which combined long-term
survey results on Bobwhites with a former GPS study of Wild Pig distribution
on a large ranch in the South Texas Plains, has identified comparative use and
selection of ecological sites common to the ecoregion by the 2 species. However,
differences in survey techniques used for the species, and inability to modify the
distribution of survey sites, resulted in an unsatisfactory gap in the data concerning
use of areas of deep sandy soils by Wild Pigs.
Although Bobwhites and Wild Pigs are to some extent both habitat generalists
(Graves 1984, Guthery 1999) and can make use of most of the ecological sites
within the rangeland, their preferred habitats were distinctly different. Wild Pigs
selected areas with clay soils, often near drainages, whereas Bobwhites were most
abundant on upland areas with deep sandy soils. The preference of Bobwhites for
sandy rather than clay-based soils is consistent with prior literature (Fulbright et al.
1990, Lehmann 1984). Sandy soils tend to support prairie with interspersed mottes
of woody plants that provide better distribution of feeding sites and cover for Bobwhites
(Guthery 1986, Hernández and Peterson 2007) than the shrub-dominated
clay and clay-loam soils. Also, unlike on clay loam soils, there was little invasion
by exotic grasses on the sandy soils. Non-native grasses provide poor brood habitat
for Bobwhites due to scarcity of insects and obstruction to movement (Sands et al.
2012). Unfortunately for this study, the area of deep sandy soil was divided from
the majority of the ranch by a highway bordered by net-wire fencing. The GPScollared
Wild Pigs never crossed the highway; thus, we could not determine the
extent to which they would have used the relatively uncommon Sandy ecological
sites favored by Bobwhites. However, the scarcity of signs of rooting by Wild Pigs
in the Sandy areas, and the low use of the Sandy Loam ecological sites, which are
Table 4. Influence of landscape features on the seasonal distribution of Wild Pigs in the South Texas
Plains Ecoregion during spring (n = 10) and summer (n = 6) 2004–2006. * indicates selected, † indicates
avoided.
% of Spring Summer
Landscape feature area % χ2 % χ2 Σ χ2 df P
Road with 20-m buffer 10.54 8.35 0.46 12.68 0.43 0.89 1 N/S
Riparian 50-m buffer from creek 10.14 27.28 28.95* 50.53 60.79* 189.74 1 less than 0.001
Water with 50-m buffer 2.08 17.90 20.05* 17.34 111.70* 231.75 1 less than 0.001
Cultivated field no buffer 2.94 22.35 128.39* 1.27 0.95 129.34 1 less than 0.001
Shrubland 82.02 31.35 31.30† 35.11 26.83† 58.13 1 less than 0.001
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similarly dry upland areas, suggest that Wild Pigs probably do not selectively use
areas of sandy soils.
The greatest overlap in habitat selection by Wild Pigs and Bobwhites occurred
early in the breeding season when Wild Pigs showed selection for Clay Loam
ecological sites. These sites are naturally vegetated with shrubland and, while not
highly preferred by Bobwhites, these extensive areas are used for nesting. However,
most locations of Wild Pigs within Clay Loam areas were not in native vegetation
but were within fallow fields that were too sparsely vegetated to be suitable nesting
habitat for Bobwhites. Accordingly, the presence of small, cultivated plots within
rangeland may reduce the impact of Wild Pigs on nesting Bobwhites by attracting
them away from the native vegetation where quail may have nests. Wild Pigs are
known to favor cultivated crop fields when available (Baber and Coblentz 1986,
Schley and Roper 2003).
By the peak of Bobwhite nesting season June–August, summer temperatures can
exceed 35 °C (NOAA 2014), during which period, the Wild Pigs spent over half of
their time in low-lying, clay-soil areas, near water and within the shaded riparian
areas, a tactic that can prevent overheating (Adkins and Harveson 2007, Graves
1984, Ilse and Hellgren 1995). Even at night when temperatures were cooler, Wild
Pigs remained predominantly in riparian areas where food resources were probably
more abundant due to higher moisture and productivity (USDA-NRCS 2012).
Riparian habitats are not the preferred nesting habitats of Bobwhites (Hiller et
al. 2007) due to the dense vegetation and risk of seasonal flooding (Klimstra and
Roseberry 1975); thus, Wild Pigs are unlikely to encounter Bobwhite nests there.
On the more mesic areas of clay-loam soils adjacent to creeks and drainages used
by Wild Pigs, dense stands of Buffelgrass provided forage and cover for Wild Pigs
but poor brood habitat for Bobwhites (Kuvlesky et al. 2002), again causing a spatial
separation of the 2 species.
In summary, in semi-arid rangeland, differences in habitat use and selection by
Bobwhites and Wild Pigs should limit interaction between the species. However,
actual rates of depredation of Bobwhite nests by Wild Pigs will be influenced by
the spatial distribution of creeks and drainages that provide these opportunistic
omnivores with travel routes into the drier upland areas preferred by Bobwhites.
Acknowledgments
Funding for this study was provided through the Reversing the Quail Decline in Texas
Initiative and the Upland Game Bird Stamp Fund based on a collaborative effort between
Texas Parks and Wildlife Department and Texas A&M AgriLife Extension Service. The
work was also supported by the USDA National Institute of Food and Agriculture, Hatch
project (accession no. 174856). We thank member ranches of the South Texas Covey Connection
for their long-lasting support and participation in the quail program. Data on Wild
Pigs was derived from a larger project supported by the US Department of Agriculture-
Cooperative State Research, Education, and Extension Service-National Research Initiative
(USDA-CSREES-NRI agreement 2003-35204-13255). We thank our collaborators J.C.
Cathey, H.M. Scott, G.R. De La Garza, A.L. Deck, A.L. Silva, and D.L. Alford for permission
to use these data.
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