UC Riverside

Given the importance of nest predation to avian fitness, ornithologists expect birds to select nest sites that minimize predation risk. Despite numerous studies contributing to a large body of literature, how predation shapes avian nest site selection is not well understood largely because studies rarely examine the processes underlying either nest site selection or predation risk. I investigated how predation shapes nest site selection for a population of Yellow Warblers with an eye for the processes underlying observed patterns. From 2000-2008, 728 Yellow Warbler nests were monitored regularly until they either succeeded or were depredated (fieldwork was collaborative with PRBO Conservation Science). Microhabitat was measured at nest sites (habitat use) and at randomly located sites (habitat availability) for analyses of habitat preference (use versus availability). I also monitored experimental nests (2006-2008) in sites that typically and atypically characterized Yellow Warbler nest sites, and I identified nest predators from video footage recorded at nests and observations of predation recorded directly by field workers.

I found conflicting evidence regarding the role of predation in shaping avian nest site selection. On the one hand, Yellow Warblers preferred high-predation nest site microhabitat patches; preferred willow-dominated patches were positively correlated with predation rates (Chapter 2). On the other hand, Yellow Warblers preferred concealment levels (> 30%) that avoided predation risk, and even dynamically changed their selection patterns to favor low-predation sites when predation pressure changed (Chapter 3). Microhabitat-predation patterns primarily arose during the egg period (laying and incubation; Chapters 2, 3, 4) as a direct result on the predatory behaviors of either avian (mainly cowbird; Molothrus ater) and/or rodent (chipmunks and mice) egg predators (identified in Chapter 1). Microhabitat-predation patterns were not confounded by coarser-scale predation patterns, which arose from different processes than did microhabitat-predation patterns (Chapter 4). These results suggest that Yellow Warblers are more capable of recognizing predator-free nest space with respect to nest concealment than microhabitat patch structure. Future research should examine the specific cues used by birds to select nest sites. Such research would likely elucidate the limitations on the ability for nesting birds to recognize predator-free space.