Hatching asynchrony and offspring sex: an experiment on maternal effects in the yellow-legged gull

Maternal effects may allow parents to increase their own fitness by adjusting progeny phenotype to pre- and post-natal conditions depending on the sex of individual offspring. In birds, sex-specific maternal effects can be mediated by predictable gradients of rearing conditions due to hatching asynchrony, egg laying date, order and mass, and brood sex ratio. In this study, we analysed the growth and survival consequences of experimentally reduced hatching asynchrony and order per se and in conjuction with other maternal effects (laying date, egg mass and sibling sex ratio) in yellow-legged gull (Larus michahellis) chicks of either sex. Survival of male chicks in synchronised broods was reduced compared to that of their sisters and that of male chicks in control broods, whereas survival of females but not that of males declined with hatching order independently of hatching synchronisation treatment. Survival of female but not male chicks declined with egg mass. In addition, survival of individual chicks, independently of their sex, was positively predicted by the proportion of females among their siblings. Hatching synchronisation resulted in larger average body size and in a weaker dependence of body mass on hatching order, suggesting that by modulating hatching asynchrony parents can effectively tune the size hierarchy among the offspring. Thus, survival of yellow-legged gull chicks of either sex was differentially affected by maternal effects as mediated by hatching asynchrony, hatching order as well as egg mass, which constitute predictable gradients of pre- or post-natal environmental conditions potentially under parental control.