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 wer e expected to refrain from increasing reproductive effort and opt for a str ategy of brood reduction, while old birds were expected to increase effort and rear more young. Increasing brood size led to an increase in foraging e ffort and brood neglect. Enlarged broods had a lower mean survival rate, bu t similar growth rates to one- and two-chick broods. Parents increased fora ging 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 ad ults increased foraging time in an attempt to satisfy the nutritional deman ds of the chicks, while older birds maintained a similar effort for all bro od sizes. Younger adults achieved a lower fledging success despite their hi gher provisioning rates. Younger birds synchronized foraging and guarding d uties less well, and so left the chicks unattended more often than older bi rds. As a consequence, their chicks suffered higher levels of conspecific p redation. Defence of chicks through pair co-ordination and aggression was t hus an important component of successful breeding for great skuas.