Abundance and rates of brood parasitism by brown-headed cowbirds over an elevational gradient in the southern Sierra Nevada

Abstract

We studied Brown-headed Cowbird (Molothrus ater) parasitism rates in four forest types (ponderosa pine, mixed conifer, true fir, and lodgepole pine) over an elevational gradient in the southern Sierra Nevada. Cowbirds were most abundant and parasitism rates were highest at the lowest sites. All but one of 17 parasitized nests were found in the ponderosa pine type and cowbirds were detected only in ponderosa pine and mixed-conifer forest types. A hypothesis that cowbird breeding and egglaying are limited by late release of livestock at higher elevations was not rejected. Data also supported a second hypothesis-that host abundance and richness influence cowbird abundance and parasitism rates. Bird species richness was a better predictor of cowbird abundance than abundance (total count per plot per year, pooled across species), and models including all passerines were better predictors than models with only host species. Brood parasitism rates were low overall, although rates for Warbling Vireos (Vireo gilvus), Cassin's Vireos (Vireo cassinii), and Black-throated Gray Warblers (Dendroica nigrescens) were high enough to warrant some concern. We recommend continued monitoring of cowbird parasitism rates for these three species in the Sierra Nevada.

Citation

Purcell, Kathryn; Verner, Jared . 1999. Abundance and rates of brood parasitism by brown-headed cowbirds over an elevational gradient in the southern Sierra Nevada. Studies in Avian Biology. 18: 97-103