Interspecific competition for nestboxes affects mate guarding in eastern bluebirds, Sialia sialis

Abstract

Abstract. Eastern bluebirds, Sialia sialis and tree swallows, Tachycineta bicolor , compete aggressively for nest cavities, and competition for cavities may force a trade-off between mate guarding and territory defence in eastern bluebirds. Behaviour of eastern bluebird pairs was examined to test (1) whether male eastern bluebirds guard their mates, and (2) whether males reduce mate guarding on territories containing multiple nestboxes, where greater interspecific competition for nest sites was found. Non-resident male bluebirds intruded onto both single-box and multi-box territories, and resident males on both territory types spent significantly more time near their mates and followed them significantly more often during the fertile period than during the non-fertile periods, supporting the hypothesis that males guard their mates to protect their paternity. Tree swallows usurped nestboxes from bluebirds on 22% of their territories during spring nesting attempts, indicating that tree swallow competition can result in displacement of bluebird pairs. Bluebirds on multi-box territories had shorter intervals between broods than bluebirds on territories with only one nestbox, but competition between eastern bluebirds and tree swallows was more intense at multi-box sites. When female bluebirds were fertile, there were significantly more intrusions by tree swallows (requiring nest-site defence by bluebirds) at multi-box territories than at single-box territories. Consequently, male eastern bluebirds on multi-box territories guarded their mates significantly less than males on single-box territories, and there was a significant correlation between the amount of time male bluebirds spent on the territory and the level of tree swallow intrusions. This study shows that there are trade-offs between mate guarding and territory defence, and that individuals within a population may adjust their mate guarding with regard to varying environmental conditions.