Factors affecting the breeding performance of the Antarctic blue-eyed shag (Phalacrocorax atriceps bransfieldensis)

Abstract

Blue-eyed Shag nestlings at two colonies on Signy Island, South Orkney Islands, have been ringed annually since 1960. By 1979 27 % of the adult population had been ringed, and were of known-age. More males than females return to the colony and breed for the first time at ages 3-4 years, and most often obtain a mate of the same age, or of one year older than themselves. Age similarities within pairs persist in older age-groups, even amongst changed pairs. In most cases the new mate is one year older than the mate of the previous season. Pair stability between successive seasons is low, and does not affect, nor is effected by, breeding success. Five nest-site characteristics were found to have no effect on breeding performance. Older males obtain nest-sites which afford a high degree of social contact with their neighbours, usually in the centre and intermediate areas of the colony. Egg laying is more synchronous than in other shag species, and there is no relationship between female age and laying date. Late clutches are usually smaller and yield fewer fledged young than early and mid-season clutches. The number of chicks hatched and f3 edged increases up to the age of 5 years, and declined after 10-11 years of ago. Clutch size and nestling survival fluctuate markedly from season to season. In clutches of three, third eggs are smaller, yield lighter chicks and usually hatch within 2-4 days of their siblings. Hatching asynchrony (rather than egg-size differences) promotes chick weight differences, a.d the early death of third chicks. The daily food consumption of young 3-chick broods is 7-8 times less than that of older, reduced broods, and it is suggested that most adults selectively starve the third chick, causing its early death. Possible advantages of this behaviour are discussed.