Breeding female cowbirds at the Queen's University Biological Station are often aggressive when encountering other females within their ranges, but they do not defend exclusive territories. The likelihood of aggression appears to be a function of two factors. First, females are more aggressive near the centres of their ranges than around its periphery, suggesting a gradient of aggression based on location within the female's range. Second, females are most aggressive in woodland and swampy areas and least aggressive in grassy areas and at artificial feeding stations. We suggest this is related to the availability of host nests.