Sex-specific traits in Common Tern Sterna hirundo chicks: associations with rearing environment, parental factors and survival

Abstract

Offspring phenotypic variation has important consequences for the future survival, reproductive success and individual fitness of nestlings, but the sex-specific reproductive value and fitness returns for parents of this variation have only rarely been explored. We analysed twelve phenotypic traits of nestling Common Terns (Sterna hirundo) in relation to their sex, age and hatching date, and explored whether sexual differences in these traits were associated with the rearing environment and survival. In addition, we studied sex-specific differences in offspring phenotype related to parental factors, since the existence of sexual dimorphism may reflect differential parental investment. Bill–head length and mediated immune response varied in relation to offspring sex, with males showing larger values than females. Immune response was also affected by environmental (year), rearing (competition over 14 days old) and parental (clutch size) factors. None of the morphological, serological and immunological variables were associated with chick mortality, but female nestlings that did not survive had suffered longer periods of within-brood competition. Although some parental traits influenced offspring phenotype, sexual differences among nestlings did not vary in relation to parental age, mass or arrival date, which may imply that there is no great difference in cost between raising sons and raising daughters, and thus no adjustment of parental effort in relation to nestling sex. However, the evidence that immune response was higher for sons in larger clutches that were raised with siblings indicates a difference in the developmental strategies for sons and daughters, which may be related to their reproductive value. These results therefore provide evidence of certain pathways for parental sex allocation that may differentially affect sons and daughters.