Egg variability and hatching success in the Cape petrel Daption capense at Nelson Island, South Shetland Islands, Antarctica

Abstract

Eggs of the Cape petrel Daption capense at Nelson Island, South Shetland Islands, Antarctica, exhibited high variability in volume between females (up to 48%), while for each individual female, both volume and shape of eggs were highly correlated over two seasons. Both female size and body‐condition at laying were unrelated to egg‐volume in the ‘good’season 1991, but larger females produced larger eggs in the ‘bad’season 1990, when eggs were smaller on average. In 1991, females in better body‐condition, but not larger, started laying earlier. In spite of synchronized laying (75% of eggs laid within five days), egg‐volume decreased with laying date. Egg‐volume differed significantly between the two years, which could not be explained by changes in the breeding population or timing of breeding. The laying of relatively small eggs and low overall breeding success in 1990 most probably reflected changes in food availability. Overall hatchability was 88% and did not differ significantly between disturbed and control colonies in 1991. Larger and more rounded eggs showed better hatching success, but hatchability was related more to egg‐volume whereas hatching rate was related more to egg‐shape. As the majority of egg losses were attributable to predation, the hatching rate was influenced by parental performance relatively more than by hatchability. Hence the relationship between hatching rate and egg‐shape probably reflects improvement in hatching success with female age/experience, whereas the relationship between egg‐size and hatchability suggests effect of egg‐size independently of parental traits.