The effect of parental age and experimentally manipulated brood size on the foraging effort and breeding performance of great skuas (Catharacta skua)

We examined the effect of a brood-size manipulation on the foraging effort and chick rearing of known-age great skuas Catharacta skua. Young birds were expected to refrain from increasing reproductive effort and opt for a strategy of brood reduction, while old birds were expected to increase effort and rear more young. Increasing brood size led to an increase in foraging effort and brood neglect. Enlarged broods had a lower mean survival rate, but similar growth rates to one- and two-chick broods. Parents increased foraging effort to maintain chick growth rates at the expense of brood defence. This was associated with higher conspecific predation of enlarged broods. Foraging effort and chick feeding rate decreased with adult age. Younger adults increased foraging time in an attempt to satisfy the nutritional demands of the chicks, while older birds maintained a similar effort for all brood sizes. Younger adults achieved a lower fledging success despite their higher provisioning rates. Younger birds synchronized foraging and guarding duties less well, and so left the chicks unattended more often than older birds. As a consequence, their chicks suffered higher levels of conspecific predation. Defence of chicks through pair co-ordination and aggression was thus an important component of successful breeding for great skuas.