Non-parental nest attendance in the barn swallow ( Hirundo rustica): helping or harassment?

Abstract

During a long-term study of barn swallows in New York's Adirondacks, we observed many instances of third birds attending active nests. Most non-parental attendance involved unmated adult males that were not closely related to those they attended. There were no significant differences in the reproductive success of attended and unattended nests. Attendants showed little of the cooperative behaviour reported in earlier studies of other barn swallow populations. At Cranberry Lake, nest attendants behaved more often in ways that were detrimental to one or both breeders (e.g. attempting copulation with females, or committing infanticide). Our results indicate that non-parental nest attendance may be a competitive stragegy by which males gain information about, or access to, breeding sites and females. If this conclusion is correct, then the major difference between barn swallows and other species with more cooperative attendant (helper) systems may be that barn swallows live, in groups of non-relatives, where there are few indirect benefits to be gained by cooperation, while more cooperative species can interact socially with kin.