Smith et til. • NESTING OF THE BLACK-CAPPED VIREO 
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ecology of Black-capped Vireos and its’ habitat 
requirements across their entire range. 
Identification of potentially critical or consis¬ 
tently present threats and habitat conditions can be 
an essential part of effective management for the 
species (Grrybowski et al. 1994). Brown-headed 
Cowbird {Molothrus ater) parasitism was w idely 
observed as a primary factor contributing to the 
lovs reproductive success of Black-capped Vireos 
at the time of listing as an endangered species 
!IJSDI 1991): cowbird abundance has shown a 
flight to moderate increase in southwest lexas 
>ince listing of the Black-capped Virco (Wilkins 
et al. 2006). Conservation and management 
efforts to mitigate high nest predation require 
knowledge of predators and predator-specific 
management (Thompson and Burhans 2003). 
Little is known about nest predators ol Black- 
capped Vireos outside of Fort Hood Military 
Reservation and surrounding areas in central 
Texas. Monitoring of nests at Fort Hood revealed 
ttim makes and imported lire ants (Solenopsis 
invicta) were the primary predators from 1998 to 
2001 (Stake and Cimprich 2003). Brown-headed 
Cowbirds and snakes were observed to be the 
primary predators of Black-capped Vireo nests in 
2008 and 2009 in the same region and further 
wuth at Kerr WMA (Conkling 2010). 
We located and monitored vireo nests in 2009 
unil 2010 to: (1) assess nest success, clutch size, 
breeding season duration, and parasitism pressure: 
f2) identify nest predators; and (3) compare our 
results with data from other regions ol the Black- 
capped Vireo's breeding range. 
METHODS 
Study Area.— Our study area was in Val Verde 
County, Texas in the Devil’s River region on the 
western edge of the Edwards Plateau (Fig. 1). 
Average rainfall from 1997 to 2008 for the 
Devil's River area during the Black-capped Vireo 
breeding season (Mar through Jul) was —5.0 cm, 
:| nd average mean temperatures ranged Irom 17.6 
C in March to 30.0 C in July (USDC 2010). Our 
Mudy area encompassed Devil s River Stale 
Natural Area (DRSNA) and Dolan Falls Preserve 
'29 53’ N, 100 59' W). a 1.942-ha property 
owned and managed by The Nature Conservancy. 
The DRSNA encompasses 8.090 ha of mostly 
unmanaged land. The property has a large 
Population of feral sheep (Ovis spp.) and aoudad 
lAmmotragus lervia), a species of sheep native to 
North Africa. There is currently no management 
of feral sheep on the property. Adjacent properties 
were undeveloped, or used lor recreation includ¬ 
ing wild game hunts, 
The natural plant communities at DRSNA and 
Dolan Falls Preserve exhibit elements of the 
mesquitc ( Prosopis spp.)-chaparral of the South 
Texas Plains, the oak-juniper ( Quercus-Juniperus ) 
woodlands of the central Edwards Plateau to the 
east, and the sotol-lechuguilla ( Dasylirion leio- 
phvltum-Agave lechugnilla) of the Trans-Pecos to 
the west (Hedges and Poole 1999). Topographic 
features of DRSNA include a nearly level plateau 
with high-domed hills and fiat-topped ridges as 
well as several large drainage systems that cut 
through canyons. Elevation ranges from 632 m at 
the northeastern park boundary to 409 m at the 
southwestern corner of the property where it 
connects to Dolan Falls Preserve (Hedges and 
Poole 1999). Dolan Creek, dry the majority of the 
year, winds 20.1 km from the north end of 
DRSNA and exits in the southwestern portion of 
the park. Dolan Creek Hows through Dolan Falls 
Preserve where it contains water year-round, the 
result of flowing springs. Topography at Dolan 
Falls Preserve is similar to DRSNA. differing only 
in that Dolan Falls Preserve contains more 
property adjacent to the Devil’s River, 
SHe Selection.—' There was little information on 
habitat use of Black-capped Vireos within the 
sampling frame, and we sampled randomly across 
(he study area which included DRSNA and Dolan 
Falls Preserve. We used ArcGIS 9.3 to create a 
grid of 1-ktrr cells lhal covered the study area and 
used Hawth’s tools (Beyer 2004) to randomly 
select four grid cells in 2009 and three in 2010. 
We initially visited each grid cell to document 
presence of Black-capped Vireos. These initial 
visits occurred no earlier than local sunrise and 
no later than 4 hrs after sunrise, 2-3 times 
during the early part of the breeding season 
(Mar and Apr): each visit was at least 4 days 
apart. We systematically walked within 200 m 
of all locations within each of the 1-knr grid 
cells, concentrating on areas with sufficient 
vegetation to support Black-capped Vireos 
(i.c., cover >20%). We recorded the coordinates 
of singing male Black-capped Vireos with a 
Global Positioning System (GPS) unit (Garmin 
Ltd., Olathe. KS. USA). We visited the cell once 
a week for a month if there were no detections 
of Black-capped Vireos after three visits to he 
certain that no Black-capped Vireos established 
territories in that cell. We randomly selected a 
