The social systems and space use of birds depend on the distribution and abundance of resources. Brood-parasitic cowbirds (Molothrus spp.) exhibit a unique pattern of changing social behavior on a daily cycle. Female Brown-headed Cowbirds (M. ater) in North America spend the morning accompanied by males searching for host nests, and in the afternoon both sexes join feeding flocks. We studied flocking patterns of Shiny Cowbirds (M. bonariensis) in open-habitat animal exhibits at a zoo in Colombia. Flock sizes of both males and females were low in the morning but increased towards noon. Male abundance remained high during the afternoon, but female abundance was very variable. Numbers of juveniles did not change throughout the day. This is consistent with the idea that cowbirds are spread out during the morning in the breeding areas and aggregate in the afternoon to feed. Cowbirds change their social behavior on a daily basis in response to the different pressures of their breeding and feeding ecologies.
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Vol. 128 • No. 2