Hatching order explains an extrapair chick advantage in western bluebirds

Within-family comparisons of offspring growth rates have provided important tests of genetic benefits of extrapair mating for females. Here, we demonstrate that hatching order explains the growth advantage for extrapair young in western bluebirds (Sialia mexicana); extrapair nestlings are larger than within-pair nestlings in the same nest, but they also hatch earlier in the clutch, thus benefiting from hatching asynchrony. By controlling for hatch order and other nongenetic factors and comparing mixed-paternity broods with genetically monogamous broods, we show that the extrapair nestling growth advantage is not genetically based. We cannot rule out the possibility that females benefit from extrapair mating because genetic quality indicators may appear later in life and may be independent of hatch order, however, based on our results, we do not see evidence of genetic superiority of extrapair nestlings. Although findings similar to ours have been attributed to maternal effects, it is currently unclear whether overrepresentation of extrapair nestlings early in the laying and hatching sequence results from investment patterns of females, their social mates, or extrapair males. This study highlights the need to investigate the potentially complex interactions among all players, and how these may lead to higher performance of extrapair offspring compared with within-pair offspring within the same family.