Behavioral Defenses to Brood Parasitism by Potential Hosts of the Brown-Headed Cowbird

Abstract

Field experiments using cowbird [Molothrus ater] models at the nests of various species were conducted. The hypothesis that potential hosts of the brown-headed cowbird use behavioral defenses of an intensity directly proportional to the intensity to which they are parasitized was tested. In general, the hypothesis was supported; within taxonomic families, a direct positive relationship exists between aggressive responses to cowbird models and the intensity of parasitism suffered. The alternative hypothesis that host aggression is used by cowbirds in nest location, and thus that the relationship between host aggressiveness and intensity of parasitism occurs for that reason is rejected. The hypothesis that the brown-headed cowbird head bowing invitation to preen posture functions as an appeasement display was tested using postured cowbird models at nests of several host species. In general, hosts were less aggressive towards cowbirds in the bowed posture than towards those in a normal posture.