Behavioral compatibility and reproductive outcomes in two distant populations of captive brown-headed cowbirds (Molothrus ater).

Increased understanding of geographic variation in courtship behavior in animal species can provide insight into the ways behavior may influence evolutionary change. Here, the relationship of geographic variation in courtship behavior to reproductive outcomes was tested in two distant and behaviorally distinct populations of brown-headed cowbirds, Molothrus ater, by comparing rates of copulation and egg production. Females from an Indiana (IN) population and a South Dakota (SD) population were housed in aviaries with males from either their own population (INsame and SDsame) or the other population (INmix and SDmix). Over the breeding season, INsame females exhibited higher rates of copulation and egg production compared with INmix females, but no differences were detected between SDsame and SDmix females. The data suggest that behavioral differences between IN and SD cowbirds are the likely cause of lower egg production in the INmix females.