Sex-biased nestling mortality is influenced by hatching asynchrony in the Lesser Black-backed Gull Larus fuscus

Many birds hatch their eggs asychronously while other species are more synchronous. Different hatching patterns may produce different sex-ratios through sex-biased chick mortality, but the relationship between sex-ratios and hatching pattern has received little attention. An extreme food shortage in a Lesser Black-backed Gull colony allowed us to examine the timing of mortality of nestlings relative to sex and to hatching order. This was carried out in experimentally-created synchronous and asynchronous broods. when the latter mimicked the normal hatching spread. Unambiguous sexing was facilitated by use of molecular methods. In asynchronous broods, last-hatched female chicks survived significantly longer than last-hatched male chicks, whilst the males significantly out-lived the females among the earlier-hatched chicks. Then was no difference in survival time of the sexes in synchronous breeds. This suggests that chick sex-ratio is, at least partly, a function of asynchrony. Production of skewed sex-ratios via sex-biased mortality should be considered when evaluating the merits of asynchrony and synchrony.