Factors affecting the breeding performance of Antarctic blue-eyed shags Phalacrocorax atriceps
The breeding performance of Blue-eyed shags of known age (up to 12 yr old) was studied on Signy Island, South Orkney Islands for three seasons. Mate change between seasons was high (77.3%), was unaffected by the age of either partner, and was unrelated to prior or subsequent breeding success. Pairs nesting in the centre of the colony experienced greater social contact, poorer access to their nests, but less exposure to wind and high seas. The degree of social contact with neighbouring nests increased with the age of the male. Average clutch sizes were of 2.31-2.84 eggs, and in one season declined with laying date. The proportion of eggs which hatched also declined with laying date, by 1.5% wk-1. Chick survival (to fledging) was higher in broods hatched in the first third of the season (88.2%) than in the last third (75.2%), and late-laying females fledged 0.64-0.66 fewer chicks on average. Some 19% of the variation in laying dates could be explained by individual consistency between seasons. In contrast to many long-lived species, there was no relationship between female age and laying date. Average clutch sizes increased after 10 yr. Eggs laid by 10-12 yr olds were 15% smaller than those of 4-9 yr olds, as were chick hatching-weights. The mean number of chicks hatched and fledged per pair increased between three and five years of age, but showed no significant change thereafter.