INTRASPECIFIC COMPETITION AND THE MAINTENANCE OF MONOGAMY IN TREE SWALLOWS

Intraspecific competition for access to breeding resources may limit male mating success in typically monogamous birds. We examined the potential for intraspecific competition to limit polygyny in tree swallows at Beaverhill Lake, Alberta, Canada. In this population, polygynous males raised more fledglings than monogamous males, and there was little or no cost to females from nesting polygynously. Under these conditions one might expect polygyny to be more common than that observed (8% of males). We found that females were most aggressive toward conspecific intruders early in the breeding season. This aggression was associated with (1) females settling farther apart than expected under random settlement, (2) later settlement by secondary than by monogamous females, and (3) no relationship between female settlement date and male territory size instead of the negative correlation expected if females settled randomly without competition. Early in the season, males also settled farther apart than expected if they had settled randomly, and among males with two or more nest boxes on their territory, males with widely separated nest boxes were more likely to be polygynous. Monogamy is probably the most common pairing association in this population because intraspecific competition for nest sites prevents most males from gaining a territory with nest sites far enough apart to permit two females to breed without one female excluding the other. Females appeared to be defending an area surrounding their nest box to limit nest usurpation or intraspecific brood parasitism, rather than to limit any loss of male parental care from polygyny.