Summary: Host use strategies of individual female brown-headed cowbirds
Molothrus ater in a diverse avian community
Bill M. Strausberger and Mary V. Ashley
Strausberger, B. M. and Ashley, M. V. 2005. Host use strategies of individual female
brown-headed cowbirds Molothrus ater in a diverse avian community. Á/ J. Avian Biol.
Although it is well established that brown-headed cowbirds Molothrus ater are host
generalists, the parasitism strategies of individual females are not well documented.
Here we use microsatellite genotyping to characterize host use by individual female
cowbirds. Of 205 offspring sampled at a site in northeastern Illinois during one
breeding season, we identified or inferred 33 mothers of 141 offspring, with up to 13
(mean 4.72) offspring assigned to individual females. Most (83%) females were host
generalists and parasitized up to five host species, indicating that community-wide
patterns of parasitism do not result from different individuals specializing on different
host species. However, some females (17%) parasitized a single species, suggesting that
alternative host-use strategies may exist within a single population. For host generalist
females, individuals often parasitized host species that differed in body mass, nest-site
placement, and quality, indicating that female cowbirds exhibit extremely flexible host
ranges. However, female cowbirds consistently discriminated among potential hosts, for
example, by completely avoiding some common, but unsuitable species.