Mortality factors and predators of Spotted Towhee nests in the Sacramento Valley, California

Abstract

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.”