Male-biased brood sex ratio depresses average phenotypic quality of barn swallow nestlings under experimentally harsh conditions.

Sex allocation strategies are believed to evolve in response to variation in fitness costs and benefits arising from the production of either sex and can be influenced by the differential susceptibility of sons and daughters to environmental conditions. We tested the effects of manipulating brood size and the sex ratio of the nestmates and the effect of sex on the phenotypic quality of individual barn swallow (Hirundo rustica) nestlings. Brood enlargement, which results in harsh rearing conditions, negatively affected the morphology and immunity of the nestlings. However, the negative consequences of brood enlargement were more marked among male than female offspring. In enlarged but not reduced broods, high proportions of male nestmates resulted in lowered individual body mass, body condition and feather growth. Thus, the consequences of a harsh environment on individual nestlings differed between the sexes and depended on the sex ratio among the other nestlings in the brood. The evolution of sex allocation strategies may therefore depend on the sex of individual nestlings but also on an interaction between environment and progeny sex ratio.