Host choice by the brood parasitic Brown-headed Cowbird (Molothrus ater) is an evolved response to host suitability,; 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 plumbeus); is heavily parasitized (51%, n = 292). To account for differences in parasitism rates on these species we experimentally; parasitized pewee nests with cowbird eggs, and we compared host aggression towards cowbird models, host nest attentiveness,; nest placement, and egg-laying dates in these species. Pewees accepted cowbirds eggs and reacted more aggressively; 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.
