Seasonal changes in brood sex composition in Audouin's Gulls

Abstract

We examined seasonal variation in the hatching sex ratio of Audouin's Gull (Larus audouinii). This species is sexually size dimorphic (males are 20% larger than females at fledging); it has a modal clutch of three eggs, which vary in size (the third egg is the smallest) and hatch asynchronously. These sex, egg size, and hatching patterns generate substantial within-brood differences in chick size that interact with the food provisioning of the parents to influence chick survival. Parental provisioning capacity depends on both parental quality and environmental conditions, both of which are known to decline with season. Consequently, the optimal brood composition is likely to vary within a season. Using molecular markers to sex newly hatched chicks, we found that offspring sex was influenced by an interaction between hatching date and hatching order, with the proportion of males among third-hatched chicks initially increasing and then decreasing later in the season.