Data from: Better-surviving barn swallow mothers produce more and better-surviving sons

Sex allocation theory predicts that parents are selected to bias their
progeny sex ratio (SR) towards the sex that will benefit the most from
parental quality. Because parental quality may differentially affect
survival of sons and daughters, a pivotal test of the adaptive value of SR
adjustment is whether parents overproduce offspring of the sex that accrue
larger fitness advantages from high parental quality. However, this
crucial test of the long-term fitness consequences of sex allocation
decisions has seldom been performed. In this study of the barn swallow
(Hirundo rustica), we showed a positive correlation between the proportion
of sons and maternal annual survival. We then experimentally demonstrated
that this association did not depend on the differential costs of rearing
offspring of either sex. Finally, we showed that maternal lifespan
positively predicted lifespan of sons but not of daughters. Because in
barn swallows lifespan is a strong determinant of lifetime reproductive
success, the results suggest that mothers overproduce offspring of the sex
that benefits the most from maternal quality. Hence, irrespective of
mechanisms causing the SR bias and mother-son covariation in lifespan, we
provide strong evidence that sex allocation decisions of mothers can
highly impact on their lifetime fitness.