THE BREEDING BEHAVIOUR OF THE SOUTH POLAR SKUA CATHARACTA MACCORMICKI

Abstract

Summary.At Cape Royds a small number of skua pairs nested at the penguin rookery and prevented other skuas from gaining any food there. Their territories were similar to those of the other pairs nesting near the rookery but which fed at sea. The birds occupied territories here from about mid‐November until early March.The first eggs were laid on 27 November and most were laid by the second week of December. Most hatched after 28–29 days incubation and all hatched within a narrow range. Male and female alternated at the nest, sharing incubation fairly evenly. Nest changes occurred at intervals of 1 1/2–3 hours. In pairs feeding at sea, the female is fed by the male during incubation.Chicks first flew when 49–59 days old but were fed by the parents for some time after this. Chicks fed on fish grew as rapidly as rookery chicks fed on penguin flesh, attained a heavier weight and grew more evenly. Length increase in primary feathers was a more useful index of growth than was gain in weight. It more nearly approached a linear relation with age and was not affected by fluctuations in feeding.Desertion and over‐cooling accounted for most of the eggs lost.Chick‐mortality fell within three categories: loss of the younger chick of the pair; loss of the older chick, or of a single chick after a period of slow growth“, loss of a single chick during normal growth. Most younger chicks were soon chased from the nest by the older chick and were killed later by cold or by adult skuas (a few were maintained by the parents for a short period away from the nest area). Only one pair reared two chicks to fledging; in other nests the older chick invariably survived longer. No chicks were lost on the territories after January.Only a small number of adult birds in any colony killed chicks and even these were only active if confined to their territories, and not able to feed at sea, through bad weather. No parents were seen killing their chicks but some ate their chicks and may have first killed them. There were numerous instances of birds adopting chicks or not harrying them. Non‐breeding birds were not important predators.The breeding success of pairs in different colonies varied from 0 to 38·5 % and averaged 23·2 % which is lower than that recorded in most other studies. Highest mortality was in territories outside colonies. This appeared to be correlated with a greater proportion of inexperienced birds in these situations. More chicks survived from rookery pairs than non‐rookery pairs. Feeding at the rookery allows both birds to be more often on the territory and there is less likelihood of food shortage during early chick‐rearing.The loss of one chick of the pair is related to the low tolerance to starvation of the older chick: a temporary food shortage causes it to chase the other from the nest area.