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1 July 2005 Mortality factors and predators of Spotted Towhee nests in the Sacramento Valley, California
Stacy L. Small
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Nest predation was the main nest mortality factor for Spotted Towhees (Pipilo maculatus) along the Sacramento River, California, during 1993–2003, followed by cowbird parasitism. From 2001 to 2003, I monitored 161 Spotted Towhee nests and filmed 24 nests using miniature infrared video cameras to identify predators. Daily survival rate was 0.928 for this period. I detected no difference in nest survival between the incubation and nestling stages. Nests with cameras had marginally higher survival rates than those without. I filmed nine nest predation events in 2002 and 2003. Eight of these were upon nestlings, while one was upon eggs. Predators were birds, mammals, and a snake, including the Red-shouldered Hawk (Buteo lineatus), Brown-headed Cowbird (Molothrus ater), Western Scrub-Jay (Aphelocoma californica), a raccoon (Procyon lotor), black or Norway rats (Rattus sp.), two unidentified rodents, and a racer (Coluber constrictor). Many of these predators are associated with agriculture and/or human habitation. I documented two multi-predator events. In both instances, a diurnal partial predation by a bird was followed by a nocturnal rodent predator. Two predation events occurred at the very end of the nestling period. I advise investigators engaged in nest success studies to consider the possibility of partial predation that may reduce a female's overall reproductive output and note that a nest should only be considered successful when direct evidence of fledglings is observed near the nest. I recommend more detailed studies of nest predator composition and the compound effects of Brown-headed Cowbird nest parasitism and “infanticide.”

Stacy L. Small "Mortality factors and predators of Spotted Towhee nests in the Sacramento Valley, California," Journal of Field Ornithology 76(3), 252-258, (1 July 2005).
Received: 28 May 2004; Accepted: 1 November 2004; Published: 1 July 2005

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Brown-headed Cowbird parasitism
nest predation
nest survival
time-lapse video cameras
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