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THE WILSON JOURNAL OL ORNITHOLOGY • Vol. 124. No. 2. June 2012 
new grid cell to replace the cells that had no 
detections after three visits. 
We established seven individual contiguous 
study sites that occurred in and around randomly- 
selected cells with Black-capped Vireos. We 
delineated study sites once we located all 
territories we were logistically capable of moni¬ 
toring. The size of each study site depended upon 
spacing of territories and the difficulty of 
monitoring territories at that particular site, which 
ranged in size from 32 to 267 ha. 
Territory Mapping and Nest Searching .—We 
mapped territories of adult male Black-capped 
Vireos and searched for vireo nests between 15 
March and 15 July. We verified that vireos were 
not exhibiting nesting behavior outside these dales 
in both years. We returned every 2 to 5 days to map 
singing locations of males to identify locations for 
subsequent nest searches. We located nests using 
behavioral cues of breeding adults (i.c.. alarm calls, 
carrying nest material, carrying food, males singing 
on the nest). We marked nests with flagging > 15 m 
and recorded locations using GPS units. 
We monitored nests every 2 to 4 days until 
nestlings fledged or the nest failed. We recorded 
the dale, lime, contents of the nest, and general 
activity (e.g., incubation, brooding, adult calls) at 
each visit to the nest. We addled any cowbird eg«s 
and removed cowbird nestlings at time of their 
discovery and recorded the parasitism. Addling 
cowbird eggs allowed the nest to remain active 
longer and allowed us to observe as many 
predation events as possible. We did not remove 
cowbird eggs from nests because presence of 
either real or artificial cowbird eggs deters future 
parasitism (Ortega et al. 1993). We searched 
territories for fledglings every 3-4 days for 
2 weeks or until a fledgling was located if we 
thought young had fledged. We considered a nest 
successful if at least one young fledged. We 
counted all parasitized nests as failures because 
parasitized Black-capped Vireo nests on average 
fledge only 0.2 fledglings per nest (Pease and 
Grzybowski 1995). 
We used nest cameras to help classify nests as: 
(1) depredated, (2) successful, or (3) parasitized. 
Nests were considered depredated if any contents 
weie removed by a predator. We categorized nests 
as parasitized if there was at least one cowbird egg 
present at any stage of the nest cycle. Nests that 
were parasitized were not considered successful 
even if nestlings survived to fledging after the 
cowbird egg was addled. 
Nest Predators .—We used Rainbow Weather¬ 
proof IR Bullet cameras (Costa Mesa. CA. USA) 
and Digital Event Recorders (DVR) (Detection 
Dynamics. Austin. TX. USA) with high capacity 
SD cards to monitor vireo nests 24 hrs ,i day 
in 2009 and 2010. Cameras were powered by a 
12-volt. 26ah battery (Batteries Plus. Hartland. 
Wl. LISA) and supplemented with a 20-watt solar 
panel (Suntech, San Francisco. CA. USA). We 
placed cameras sufficiently close to nesLs to 
observe activity but not so close as to disturb 
the birds (1-2 m). Cameras were attached to a 
DVR. battery, and solar panel by a 15-m cable so 
the system could be maintained without disturbing 
the nest. Cameras were evenly distributed among 
each of the study sites monitored each year. We 
placed cameras as early as possible in the nesting 
cycle, but only after incubation had begun to 
avoid abandonment of the nest (Stake and 
Cimprich 2003). We observed the nest until adults 
returned and removed the camera if the adults did 
not return within 30 min. We randomly chose a 
new nest location within each study site to deploy 
the camera to monitor as many nests as possible 
once young fledged or the nest failed. 
Statistical Analyses. - We evaluated nest suc¬ 
cess using the Mayfield Method (1961. 1 975) and 
Program MARK to calculate daily nest survival. 
We only used data from nests in which eggs or 
nestlings were observed in calculating daily 
survival rate. We used SAS 9.2 (SAS Institute, 
Cary. NC. USA) for statistical analyses. We 
calculated mean and standard error by year tor 
clutch size and host eggs hatched, and compared 
those means using /-tests (Zar 1999:122-129). We 
performed Chi-square analyses to ascertain it 
parasitism, depredation, clutch size, or proportion 
ol eggs that hatched were statistically different 
between years. 
RESULTS 
We located and monitored 81 and 38 nests in 
2009 and 2010, respectively. The earliest dated 
incubation was 14 April 2009 and 7 April 2010. 
despite early nest building in mid-March in 2004 
1 lie mean (± SD) monthly rainfall was 3.6 - 
2.9 cm in 2009. 1.1 cm per month below norma 
and 7.0 cm above normal in 2010. averaging I 
± 10.1 cm per month from March to July. Regular 
flooding ol Dolan Creek and other typically dry 
drainages throughout the study area occurred in 
2010. The average daily temperatures in 2009 
were above normal, ranging from 18.6 C in 
