Addition of arthropod cocoons to house wren nests is correlated with delayed pairing

Males in the cavity-nesting house wren (Troglodytes aedon) frequently add arthropod cocoons to their nests during building, possibly as an ornamental cue for female choice. We tested this hypothesis by comparing the time to pairing for males that did and did not add cocoons to their nests and for males in whose nests we manipulated the number of cocoons prior to pairing. We also tested the hypothesis that females acquire fitness-related benefits by selecting mates based on their use of cocoons. The use of cocoons by males was not consistently related to habitat, but the number of cocoons added per nest increased during the course of the breeding season. Contrary to prediction, the time to pairing for males adding cocoons was significantly longer than that for males without cocoons in their nests at both unmanipulated and experimental nests. There was also no consistent fitness-related benefit for females related to the use of cocoons by their mates. Therefore, we conclude that females did not prefer males that added cocoons to their nests, and that the increased time to pairing for males that add cocoons likely results in fitness-related costs brought about by delayed breeding. Nonetheless, male house wrens routinely use cocoons, and why they do so remains unknown. Copyright 2005.