Cowbirds as a model system

The brown-headed cowbird serves as an excellent model to explore how social environments can organize and control behavioral development because the species experiences a wide variation in social ecology. Being brood parasites, cowbirds spend their early life unexposed to species-typical behavior, but once they fledge, they join cowbird flocks that can vary extensively in age and sex composition. Because cowbirds have been so successful in expanding their range, they are found in a variety of different ecologies. They can be found in most parts of North America and they have been reported to parasitize many different species (more than 200 different host species) in many types of habitat. Across their range, cowbirds can experience differences in population density, climate, sex ratio, and timing of the breeding season. Different groups show regional differences in song dialects and whistles; some populations migrate, some do not. Adults and juveniles may migrate different distances. Variation also exists in reports of cowbird mating systems, which can range from promiscuity with no pair bonds, to polygyny, polyandry, and monogamy, the latter of which seems to be the most prevalent. In sum, cowbirds experience myriad environments. In some locations, throughout their first year, juveniles may never interact with adults, while in other locations, juveniles join flocks of other cowbirds while adults are still in the final days of breeding and remain with them for the entire year. Flocks composed of all females, all juveniles, and all classes have been reported in the field. Males use whistles and songs to establish and indicate dominance as well as to stimulate females. Females use visual signals such as wing strokes and beak movements to interact when song occurs. There are few species in which so much is known about this proximate level of social interaction.