Decline of the breeding population of <Emphasis Type="Italic">Pygoscelis antarctica</Emphasis> and <Emphasis Type="Italic">Pygoscelis adeliae</Emphasis> on Penguin Island,South Shetland,Antarctica

This work is a quantitative analysis of the population of breeding pairs of Pygoscelis adeliae and Pygoscelis antarctica, which use Penguin Island as a breeding area. There was a decrease in the populations of P. adeliae and P. antarctica by 75 and 66%, respectively, from the breeding season of 1979/1980 to 2003/2004. A mixed breeding colony of both species was also documented, according to published data which has existed since 1979/1980. The P. antarctica has an advantage over P. adeliae in this mixed breeding situation, where the breeding pairs of the former increased by 127%, indicating an interspecies competition for nesting ground.