Differential Parasitism Between Two Suitable Cowbird Hosts

Abstract

Host choice by the brood parasitic Brown-headed Cowbird (Molothrus ater) is an evolved response to host suit- ability, resulting in patterns of differential parasitism rates among species within a community. In the ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range, we recorded that Western Wood-pewee (Contopus sordidulus) is infrequently parasitized (1%, n = 259 nests) by the Brown-headed Cowbird, whereas the Plumbeous Vireo (Vireo plum- beus) is heavily parasitized (51%, n = 292). To account for differences in parasitism rates on these species we experimen- tally parasitized pewee nests with cowbird eggs, and we compared host aggression towards cowbird models, host nest at- tentiveness, nest placement, and egg-laying dates in these species. Pewees accepted cowbirds eggs and reacted more ag- gressively towards the cowbird model than the control model, were more attentive at their nest sites than vireos, and placed their nests higher and closer to the trunk than vireos. Egg-laying dates for vireos and cowbirds overlapped more than the egg-laying dates for pewees and cowbirds. We suggest that temporal asynchrony in host availability, coupled with differences in nest placement and behavior at the nest, help to account for the observed differences in parasitism rates between these two species.