Differential Parasitism Between Two Suitable Cowbird Hosts

John J. Walsh1, Ty A. Tuff1, Alexander Cruz1, Jameson F. Chace2, *
1 Department of Ecology and Evolutionary Biology, Campus Box 334, University of Colorado, Boulder, CO 80309-0334, USA
2 Department of Biology and Biomedical Sciences, Salve Regina University, Newport, RI 02840, USA

© 2015 Chace et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: ( This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Department of Biology and Biomedical Sciences, Salve Regina University, Newport, RI 02840, USA; Tel: 401-341-3204; Fax: 401-341-2993; E-mail:


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.

Keywords: Brood parasitism, brown-headed cowbird, Contopus sordidulus, Molothrus ater, plumbeous vireo, Vireo plumbeus, western wood-pewee.