Faculty Research & Creative Activity

Document Type


Publication Date

January 2006


The relationship between the Brownheaded Cowbird (Molothrus ater) and its cavitynesting hosts has received little attention because of the assumption that cowbirds rarely parasitize these hosts. We tested the Eastern Bluebird (Sialia sialis), a host that is sometimes heavily parasitized by cowbirds, for egg ejection behavior. Bluebirds ejected 65% of experimentally added cowbird eggs (n = 20), but ejected no experimentally added conspecific eggs (n = 66). This suggests that cowbird parasitism, not conspecific brood parasitism, is the selective pressure responsible for egg ejection in this species. This level of rejection may be conservative because bluebirds nest in dark cavities, which may make cowbird eggs difficult to detect by bluebirds.


Published as Peer, B. D., L. R. Hawkins, E. P. Steinke, P. B. Bollinger, and E. K. Bollinger. 2006. Eastern Bluebirds eject Brown-headed Cowbird eggs. Condor 108:741-745.. © 2006 by the Regents of the University of California/Sponsoring Society or Association. Copying and permissions notice: Authorization to copy this content beyond fair use (as specified in Sections 107 and 108 of the U. S. Copyright Law) for internal or personal use, or the internal or personal use of specific clients, is granted by the Regents of the University of California/on behalf of the Cooper Ornithological Society for libraries and other users, provided that they are registered with and pay the specified fee via Rightslink® on JSTOR (http://www.jstor.org/r/ucal)] or directly with the Copyright Clearance Center, http://www.copyright.com.