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To explore the costs and benefits of groups living in a facultatively social bird, we studied the reproductive ecology of a marked population of Barn Swallows (Hirundo rustica) at the Cranberry Lake Biological Station in the Adirondack Mountains of New York State.Throughout its range, the Barn Swallow breeds solitarily or in small groups. From 1979 through 1985 we gathered reproductive and mortality data in colonies varying in size and density. Birds living in larger colonies tended to be younger and to breed later, on average, than solitary breeders. Breeding success was less for birds in large colonies than for birds breeding solitarily or in small groups. Larger colonies did not provide obvious benefits that offset the reduced reproductive success.Nests in larger colonies were infested more by a bloodsucking blow fly larva (Pratocalliphora hirundo) than were nests in smaller colonies. The fraction of nests infested and the mean number of parasites per nestling also increased as the breeding season progressed. Since parasitism appeared to be the major source of nestling mortality, the increased parasite loads were probably the major cost of living in larger groups in our area. Birds in large colonies suffered a net reproductive loss relative to those breeding alone or in smaller groups.Thus, our data are consistent with Snapp's (1976) conclusion that shortages of nesting habitat rather than direct benefits of group living are responsible for swallow coloniality.

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