Survival of extrapair and within-pair young in tree swallows

In monogamous species, it is generally accepted that males seek extrapair matings to increase their reproductive success without additional parental investment; however, the benefits of extrapair matings to females are much less clear. One possibility is that females obtain genes for enhanced offspring viability from the extrapair sires. If this is the case, then the increased viability of extrapair young may be evident throughout the period of embryonic development as well as later in life. Tree swallows (Tachycineta bicolor) have one of the highest known levels of extrapair mating in birds, and females have substantial control over the paternity of their offspring. We used molecular techniques to determine the parentage of nestlings and unhatched embryos to examine the possibility that female tree swallows gain viability benefits for their extrapair offspring. Although both extrapair paternity and mortality of embryos and nestlings were high (89% and 54% of broods respectively), we found no difference in the viability of within-pair and extrapair young prior to fledging. In addition, extrapair young were not more likely to be male. There was no bias in the sex of young at fledging, but unhatched embryos were more likely to be male. Our results do not support the idea that female tree swallows engage in extrapair mating to increase offspring viability, at least early in life.