Sex-specific vulnerability to breeding conditions in chicks of the sexually monomorphic Gull-billed Tern

Environmental conditions during early development may differentially affect male and female offspring, and the effects of this sex-environment interaction in chick performance may be exaggerated under harsh conditions. In birds, most of the currently available evidence on sex-biased environmental sensitivity in nestlings is derived from species that display sexual size dimorphism, while studies on monomorphic or slightly dimorphic species are less abundant and have produced inconsistent results. We have evaluated sex-specific vulnerability to breeding conditions in chicks of the Gull-billed Tern (Gelochelidon nilotica), a semiprecocial species with only low sexual size dimorphism. We compared male and female mass growth and fledgling physiological condition (measured through plasma metabolite levels) in several colonies that differed in reproductive parameters. Chicks of both sexes grew more slowly and fledged with lower mass and poorer nutritional state in the colony with the worst breeding conditions, i.e., with later phenology and lower clutch size and reproductive success. Contrary to our expectations, chick vulnerability to rearing conditions was more pronounced for female than male fledglings. While males grew faster than females during the middle phase of growth regardless of colony, this difference disappeared later in the fledging period in all but the worst colony, where females maintained a lower mass and worse nutritional condition than males. These results add to the evidence that, even in monomorphic species, the environmental sensitivity of nestlings during development may vary in a sex-specific way that may select for sex-biased allocation of parental resources and sex ratio adjustments under specific breeding conditions.