| Summary: | At Laurie Island, South Orkney Islands, there are some breeding areas where two Pygoscelid species (Pygoscelis adeliae and P. antarctica) breed at the same sites, competing for the available space. To study the effects of this competition on the breeding performance of the Adèlie penguin, data on breeding chronology, population size and reproductive success were collected from the 1995/96 to 1997/98 breeding seasons in areas of competition (mixed groups) and areas where only Adèlie penguins breed (single groups). The numbers of Adèlie breeding pairs increased by 54% in single groups while they dropped by 14% in mixed groups. The number of chicks in creches followed a similar overall trend in mixed and single groups throughout the study period. However, while the chicks in creches in mixed groups represented 64% of the total number in 1995, they only amounted to 56% in the 1997 season. On average, 32% of the Adèlie nests were usurped by P. antarctica. The peak of Adèlie nest losses to chinstraps occurred from 6–20 November in 1995 and 1996, while in 1997 the peak of nest site takeovers took place between 20 November and 6 December due to the later arrival of chinstraps. In areas of competition, the overall breeding success of Adèlie penguins was significantly depressed, this depression being due mainly to egg losses. We suggest that the presence of chinstraps had not only direct adverse effects on the breeding success of Adèlie penguins in mixed sites, but also a possible long-term effect by negatively impacting on the recruitment of pre-breeding birds.
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