Siblicidal brood reduction in South Polar Skuas

Abstract

Siblicide in the South Polar Skua (Catharacta maccormicki) was studied at Cape Crozier, Ross Island, Antarctica during the 1993/94 austral summer. Sixty‐six two‐egg nests were monitored, providing 49 two‐chick broods for study. Eighteen pairs of chicks (37% of broods) were raised. Single chicks survived in 27 nests, of which 26 were first chicks. Only three second chicks outlived firsts in the same brood, of which only one survived to at least 20 days. Attacks began within the first days of the second chick hatching. Second chicks were lost or evicted from the nest area on average 3.05 ± 0.5 days (mean ± SE) after hatching. Although first and second chicks had almost identical body masses at hatching (68.3 ± 4.7 g versus 69.9 ± 5.6 g), first chicks weighed 102.3 ± 4.2 g when the second hatched. At the peak of attacks at 48 h after the second hatched, first chicks weighed 146.5 ± 4.7 g on average and seconds only 83.3 ± 1.8 g. Between‐brood differences in mass and age of chicks in different broods had minimal influence on either the intensity of sibling aggression or its outcome. First chick aggression towards the second persisted for at least 3 weeks in broods kept separately in enclosures at the nest. All attacks began near the nest, usually at chick feeding. Parents intervened in a high proportion of attacks, invariably in pecking attacks, but did not halt them in the longer term. Twelve of 26 second chicks survived for a time elsewhere on the territory or were adopted by neighbours after being evicted from the nest area. Brood reduction through siblicide in this species is a two‐phase process: eviction from the nest area, followed by death from predation, exposure or starvation. A review of the literature showed that varying levels of siblicide occur throughout the entire breeding range of the South Polar Skua, and, except for some possible records for the Brown Skua, is uniquely confined to this species within the Stercorariidae. We concluded that brood reduction through siblicide is facultative, its intensity in each population each season determined by local foraging conditions.