Male extrapair nestlings fledge first

Various studies have reported that extrapair young outperform their within-pair half-siblings in fitness-relevant traits, suggesting indirect benefits of extrapair copulations for females. Recent studies, however, suggest that potential confounding maternal effects such as laying and hatching order may have been overlooked. In this study on blue tits, Cyanistes caeruleus, we investigated fledging behaviour, a potentially fitness-relevant trait, in relation to the paternity status of the offspring. We tested whether extrapair young differed from within-pair young in fledging order per se and after we controlled for hatching order, a potential confounding maternal effect. We found that male, but not female, extrapair young fledged earlier than their within-pair nestmates. This effect was partly mediated by the higher body mass of extrapair young, which is a major determinant of fledging order independent of paternity status. However, even when we controlled for body mass, a correlate of hatching order, male extrapair young still fledged earlier than their same-sex half-siblings, indicating that male extrapair nestlings may be at an advantage over their siblings.