Survival and Mortality in a Population of Adelie Penguins

Abstract

Life table and survivorship estimates were calculated for Adelie Penguins (Pygoscelis adeliae) of known age, sex, and breeding status. Fledglings had been banded and web—punched in nine consecutive breeding seasons at Cape Crozier, Ross Island. During four later seasons the returns of 2— to 14—yr—olds were recorded. Adjustments for band loss and emigration were made in the data. Survivorship estimates were also calculated for the Yellow—eyed Penguin (Megadyptes antipodes) using Richdale's (1949, 1957) data. Results indicate that breeding is risky, especially in the Adelie Penguin and particularly among younger breeders. In both species, females breed at younger ages than males and in any mature age class a greater proportion of females than males breed. These factors result in higher mortality among females and a sex ratio that changes to favor males in the older age groups. The oldest individuals tend to be those that are habitually inept breeders or nonbreeders. The higher annual mortality and shorter life—span in the Adelie Penguin compared to the Yellow—eyed Penguin are probably due in part to heavier predation pressure.