Habitat associations and nest survival of yellow warblers in California

Yellow Warblers have experienced population declines in California, earning them special status as a Species of Special Concern. The causes are thought to be habitat loss, nest predation, and Brown-headed Cowbird parasitism. In order to effectively conserve their remaining populations it is imperative to understand their specific habitat requirements and susceptibility to predation and parasitism. Ecological factors that best explained the distribution of Yellow Warblers were investigated by conducting point counts and recording stream and landscape, vegetation, and predator and parasite characteristics along streams in Santa Cruz County, California. In addition, predation and parasitism pressures were examined by monitoring nests and determining reproductive success. Yellow Warblers were highly associated with agriculture on the landscape scale. On the patch scale, willow (Salix sp.) shrubs and stream characteristics that are conducive to willow growth were the best predictors of Yellow Warbler presence at a site. A notably large portion of the Yellow Warblers breeding in the study area was found along the Pajaro River, a stream that is leveed and managed for flood control through annual vegetation-reduction regimes. However, the Yellow Warbler's partiality to this heavily disturbed system was met with very low nesting success due to high predation rates and cowbird parasitism, indicating that this scenario may be an ecological trap.