Population Dynamics of Breeding South Polar Skuas of Unknown Age

Abstract

SEVERAL authors have presented data and discussed various aspects of population dynamics of the two most southerly breeding forms of Catharacta, the South Polar Skua (C. maccormicki) (Eklund, 1961; Young, 1963; LeMorvan et al., 1967; Pryor, 1968) and the Brown Skua (C. skua lonnbergi) (Stonehouse, 1956; Burton, 1968; Purchase, in Carrick and Ingham, 1970). These studies have been either of short duration, based on few breeding pairs, or both. Breeding success in the extreme antarctic environment varies highly from year to year and also considerably within a region of dissimilar topography. Survival is less variable, but a large population must be studied to obtain accurate figures. This study of individually marked skuas of unknown age took place at Cape Crozier (770 27' S, 169? 14' E) on Ross Island, Antarctica during eight consecutive breeding seasons (November through February) beginning in 1961. It forms a base line for current studies on breeding skuas of known age. The South Polar Skua population at Cape Crozier consists of 950 to 1,000 pairs of breeding birds and an estimated 200 to 400 nonbreeders. Approximately 80 per cent of each group was banded. Six discrete breeding areas are dispersed along the inland periphery of a rookery of 175,000 pairs of Adelie Penguins (Pygoscelis adeliae) (Emison, 1968). One breeding area of approximately 11 hectares (27 acres) was chosen at the beginning of the study for its relative freedom from human disturbance and for its isolation from other skua breeding areas. Topography varied from nearly level to a slope of 42 per cent. During 5 consecutive years of the study the mean number of breeding pairs was 110 (Table 4); 94 per cent of the breeders were banded and of these 55 per cent were sexed by copulatory position and/or by courtship feeding (Young, 1963). Breeding skuas on their territories were caught with a hand net when they dived at the observer's head (Austin, 1957). Breeding skuas away from their territories and nonbreeders were baited and noosed. The only banding casualty was a bird whose wing was broken when it was netted. Three types of bands were used. All captured skuas were given a 25-mm (1 inch) tall aluminum band with the serial number imprinted vertically on two opposite sides of the band to facilitate reading with binoculars or spotting scope (Sladen, Wood, and Monaghan, 1968). This band was placed on the right tarsus of adult skuas and on the left