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Article Excerpt
Survival of migrant songbirds is generally thought to be high during the breeding season (Sillett and Holmes 2002): however, there is little direct evidence of the frequency or sources of adult mortality during this period. Documenting any predation event on the nest is difficult (Pettingill 1976) and adult mortality is usually inferred from remains left at the nest (Sherry and Holmes 1997, Flaspohler et al. 2001). Often evidence suggests that adults were killed in defense of the nest or circumstantially, and were not the intended victim (Fendley 1980, Quinn 1985, King 1999). Interpretation of evidence at the nest, however, can be an unreliable method to identify nest fate, cause of nest failure, or identity of a predator (Thompson et al. 1999).
Video surveillance has recently been shown to be an effective and reliable method to identify and evaluate the importance of nest predators (Thompson 2007). The main groups of nest predators identified from these studies were sciurids, corvids, raptors, and snakes. Adult mortality was only observed in two studies using video surveillance to monitor songbirds--a thirteen-lined ground squirrel (Spermophilus tridecemlineatus) depredated one Chestnut-collared Longspur (Calcarius ornatus) female (Pietz and Granfors 2000) and rat snakes (Elaphe spp.) depredated three Golden-cheeked Warbler (Dendroica chrysoparia) females (Stake et al. 2004). The potential for snake predation of incubating females has long been speculated (Laskey 1946, Mahan 1956), but this phenomenon has rarely been documented (Plummer 1977, Blem 1979, Carter 1992). We believe snakes pose the greatest risk of depredating adult songbirds at the nest because they are the only group known to be significant nocturnal predators, at least in some habitats (Hensley and Smith 1986, Stake and Cimprich 2003, Stake et al. 2004, Reidy et al. 2008), large enough to kill and consume adult songbirds.
We monitored Golden-cheeked Warbler nests with video cameras and report on causes and rates of nest mortality, and identity of nest predators elsewhere (Stake et al. 2004, Reidy et al. 2008). Our objective in this study is to report on the extent of adult mortality at the nest and discuss its potential implications. Golden-cheeked Warblers, a federally endangered species, are endemic breeders in central Texas with strict nesting habitat requirements (Ladd and Gass 1999). The requisite mixed mature Ashe juniper (Juniperus ashei) and oak (Quercus spp.) woodlands in which they nest are currently being heavily fragmented and developed in historically important parts of the breeding range (USDI 1992).
METHODS
Study Area.--We monitored nests of Golden-checked Warblers at Fort Hood Military Reservation, Texas, USA (30[degrees] 10' N, 97[degrees] 45' W) from 1997 to 2002 and in 2005, and in Austin, Texas (30[degrees] 23' N, 97[degrees] 34' W) from 2005 to 2006. Fort Hood (Bell and Coryell counties) is a large, active military base in a rural landscape with discrete patches of Golden-checked Warbler habitat separated by open valleys leased for cattle grazing. Austin (Travis County) is a large and growing city with historically large amounts of contiguous breeding habitat currently being fragmented by human development.
Field Methods.--We mapped territories of banded and unbanded adults from March to May to establish pairing and nesting status of males, and to narrow the nest-searching area. We searched for nests from March to June, typically using adult behavioral cues to locate nests. We placed miniature video cameras with infra-red illumination (Fuhrman Diversified Inc., Seabrook, TX, USA) that allowed continuous monitoring at as many nests as possible during the incubation and nestling stages, prioritizing nests in the incubation stage. Video cameras were attached by a 20-m long cable to a video recorder and battery placed as far from the nest as possible. We monitored nests daily using a monitor that plugged into the video recorder and did not approach the nest while it...
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