Local gradients of cowbird abundance and parasitism relative to livestock grazing in a western landscape

We studied local patterns of Brown-headed Cowbird (Molothrus ater) abundanc e parasitism rates and nest success of a common host the Plumbeous Vireo (V ireo plumbeus), in relation to the distribution of livestock grazing in an undeveloped region of northeastern New Mexico, 1992-1997. We predicted that both cowbird abundance and parasitism rates of vireo nests would decrease with inn-easing distance from active livestock grazing, and that the nestin g success of vireos would increase. We measured cowbird abundance and host density and located and monitored vireo nests in pinyon-juniper and mixed-c onifer habitats that ranged from actively grazed to isolated from livestock grazing by up to 12 km. Cowbird abundance declined with distance from acti ve livestock grazing and was not related to host density or. habitat type. Brood parasitism levels of vireo nests (n = 182) decreased from >80% in act ively grazed habitats to 33% in habitats that were 8-12 km from active graz ing but did not vary by habitat type or distance to forest edge. Vireo nest ing success was higher in mixed-conifer habitat than in pinyon-juniper but was unrelated to distance from active livestock grazing. Nest losses due to parasitism declined with distance from active livestock grazing. Our resul ts suggest that cowbird abundance and parasitism rates of bests may be dist ributed as a declining gradient based on distance from cowbird feeding site s and that isolation from feeding sites can reduce the effects of parasitis m on host populations. These findings provide support for management techni ques that propose to reduce local cowbird numbers and parasitism levels by manipulating the distribution of cowbird feeding sites. The presence of par asitized nests >8 km from active livestock grazing suggests that in some re gions, management efforts may need to occur at larger scales than previousl y realized.