Current Status of the Brown-Headed Cowbird in the Sierra National Forest

Seasonal distribution and abundance, habitat preferences and use, and host relationships of Brown-headed Cowbirds (Molothrus ater) in the Sierra Nevada of California were studied in the Sierra National Forest in 1980 and 1981. From several hundred counts, including many made in 1978 and 1979, the relative numbers of cowbirds and potential hosts were estimated for a variety of habitats and for varying distances from known loci of afternoon foraging aggregations. Intensive searches were made for nests and fledged broods of potential hosts. Cowbirds strongly prefer meadow edges as breeding habitats, using clearcuts and incompletely logged forests to some extent and unlogged forests rarely or not at all. In the preferred meadow habitats, cowbird abundance declined rapidly with increasing distance from human-based sources of food, such as pack stations. The relative abundance of Warbling Vireos (Vireo gilvus) was negatively correlated with that of cowbirds, supporting other evidence of high rates of parasitism on this host species in the Sierra Nevada. Because cowbirds are rare in or absent from many major habitat types and areas remote from human-based sources of supplemental food, however, we doubt that any species in the Sierra Nevada is currently threatened by cowbird parasitism. The problem needs periodic monitoring, however, as human developments become more widespread in the mountains.