Brothers and sisters are stabbing each other in the back: long-term effects of sex of siblings on barn swallow offspring

Prenatal and early postnatal conditions can prime developmental trajectories, with short- as well as major long-term effects on phenotype. Variance in perinatal conditions may be caused by many factors, including number of siblings of either sex, which is expected to influence individual phenotype for two complementary reasons. First, male and female offspring can differ in susceptibility to extrinsic conditions. Second, the effect that an individual has on its siblings can differ according to their sex. Yet, few studies of vertebrates and only one of birds have addressed the long-term consequences of family sex composition on offspring of either sex. We analysed the effect of brood sex composition on adult phenotype and breeding performance in the barn swallow, Hirundo rustica. Male offspring growing with more sisters had shorter wing length, an aerodynamically important trait, and tail length, a condition-dependent, sexually selected trait. In addition, tail length of females decreased with increasing brood size but more steeply so when they grew with more female siblings. Body size of females also declined with increasing brood size. Notably, breeding output of females declined with an increasing proportion of male siblings and also with increasing brood size. This study thus suggests that social environment has major consequences for phenotype and breeding performance in adulthood and that variation in brood sex composition has long-term effects which depend on the sex of the individual as well as on the specific trait considered. Hence, optimal parental sex allocation decisions depend not simply on additive fitness costs and benefits of producing males or females, but also on the long-term effects that sons and daughters exert on each other.