Parasitism by Brown-Headed Cowbirds in the Shrubsteppe of Eastern Washington

Shrubsteppe communities within the Intermountain West have been reduced in area and fragmented by agricultural conversion and land development, yet we know little about the effects of Brown-headed Cowbirds (Molothrus ater) on reproductive success of birds that breed in these communities. As part of ongoing research examining landscape effects on avian productivity in eastern Washington, we collected data on parasitism rates and cowbird occurrence. During 1996 and 1997 we surveyed birds using point-counts and searched for nests in big sagebrush (Artemisia tridentata) stands in eastern Washington. Cowbirds were common on our study area and were recorded on point-counts at 26 of 29 sites surveyed. Cowbirds arrived on the study area in late April, attaining greatest abundance in May and June. We located and monitored a total of 779 nests of 8 species; only the Brewer's Sparrow (Spizella breweri), Sage Sparrow (Amphispiza belli), and Vesper Sparrow (Pooecetes gramineus) showed evidence of parasitism. Overall parasitism rates were lower than those reported for other bird communities in fragmented landscapes and for other bird communities in shrubsteppe. Low parasitism levels (<10 %) in our study area partly resulted from arrival of cowbirds after initiation of first nests by hosts. Over 40% of Sage Sparrow nests were initiated before cowbirds were observed laying on the study area. Low levels of parasitism also may be related to low availability of elevated observation perches or long distances from study plots to cowbird feeding areas. Determining why parasitism is low in this fragmented landscape may have important implications for managing cowbirds in other areas.

Parasitism by Brown-headed Cowbirds (Molothrus ater) has been found to depress significantly the reproductive output of some passerines, particularly in fragmented landscapes (Brittingham and Temple 1983, Robinson et al. 1995). Shrubsteppe communities within the Intermountain-west have been reduced in area and fragmented by agricultural conversion and land development (Quigley and Arbelbide 1997), particularly within the Columbia River Basin in eastern Washington (Dobler et al. 1996). Moreover, these communities have a long history of use as rangeland, providing feeding habitat for cowbirds in the form of feedlots, pastures, and lawns. A recent analysis of data from the Breeding Bird Survey for the Columbia River Basin reported significant, declining trends for populations of numerous shrubsteppe-associated species, with more species declining than increasing (Saab and Rich 1997). We know little about the effects of cowbirds on reproductive success of birds that breed in shrubsteppe communities (Rich 1978, Reynolds 1981, Rich and Rothstein 1985, Biermann et al. 1987).

As part of an ongoing research project examining landscape effects on avian productivity in eastern Washington, we collected data on parasitism levels and cowbird occurrence. Here we present a preliminary assessment of cowbird parasitism on the more common nesting passerines in Washington's shrubsteppe.