The comparative breeding biology of Adelie and Chinstrap Penguins Pygoscelis adeliae and P. antarctica at Signy Island, South Orkney Islands
Adélie Penguins and Chinstrap Penguins breed in abundance at Signy Island, South Orkney Islands, which is near the northern breeding limit of the former but in the centre of the latter's breeding range. During the 1980–81 and 1981–82 breeding seasons daily checks were made to record laying date and interval between laying, egg weight and volume, incubation period and shifts by each parent, hatching date and interval between hatching, the date on which the young first creched and their age at the time, and fledging date and age. This was done for the eggs and chicks from 60 marked nests of each species. Adelies started breeding about one month before Chinstraps and showed other adaptations which may relate to early breeding and probably to the presence of ice around the breeding site. These adaptations included longer incubation shifts and a heavier and larger first egg which may promote more rapid brood reduction. Chinstraps had shorter incubation shifts and both eggs had similar weights and volumes. Although the later breeding of Chinstraps meant that chick rearing coincided with the availability of a better food source than that utilized by Adelies, Chinstraps were unable to cope with persistent heavy ice conditions in 1980–81 and were no more successful than Adelies in 1981–82. All previous breeding data for both species are reviewed and it is suggested that early breeding by Adelies may be a consequence of competition for food with Chinstraps but, by retaining adaptations to harsh environmental conditions, Adelies are well able to breed successfully. Conversely, Chinstrap Penguins appear mainly adapted to milder environmental conditions and it is unlikely that they could compete successfully with Adelie Penguins at breeding sites on the Antarctic Continent.