| Extra-pair paternity is common among socially monogamous birds, yet our understanding of the factors influencing female selection of social and extra-pair mates, as well as the fitness consequences of extra-pair fertilizations, remains limited for many species. Using a combination of nest-site manipulations and paternity analyses, I studied tree swallows (Tachycineta bicolor) breeding in central British Columbia to examine (1) the relative importance of nest-site and male quality for selection of social mates by females, (2) phenotypic characteristics associated with extra-pair mating success in resident males, and (3) the consequences of extra-pair paternity for offspring performance.;In one of the two study populations, birds were less likely to occupy nest boxes that had a high perceived risk of ectoparasitism, suggesting that choice of social mates in this population is influenced by nest-site quality. In both populations, however, female tree swallows produced smaller clutches in high-risk (treatment) nest boxes. This difference was not explained by phenotypic characteristics of females or their social mates, indicating that nest-site quality, though not consistently influencing settlement decisions, may have important effects on primary reproductive investment of female tree swallows. In contrast, male quality was associated with extra-pair mating success: males who sired extra-pair offspring had longer flight feathers, were more likely to be returning breeders, attracted social mates who tended to breed earlier, and raised nestlings with higher growth rates of ninth primary feathers compared to males producing only within-pair offspring. These results support the hypothesis that extra-pair mating allows female tree swallows to obtain good genes for their offspring. Additional studies are needed to examine the extent to which flight feather length varies independent of male age in tree swallows, and thus, whether this trait may be influenced by sexual selection. Finally, comparisons of maternal half-siblings showed that extra-pair offspring grew their ninth primary feathers faster under all conditions, but differences in length of ninth primaries at fledging were only evident under low-stress conditions, when extra-pair offspring were positioned early in the hatching sequence and the intensity of blow fly (Protocalliphora spp.) parasitism was low. Within-pair offspring exposed to favourable conditions did not show a similar advantage. This indicates that genetic benefits of extra-pair paternity depend on the environmental conditions to which nestlings are exposed, and extra-pair mating may therefore represent a 'bet-hedging' strategy in response to environmental unpredictability. Consequently, I suggest that tests for genetic benefits of extra-pair fertilizations in socially monogamous species should consider the potential influence of environmental conditions on the phenotypic expression of genetic variation in offspring. |
