FACTORS AFFECTING THE DISTRIBUTION AND SIZE OF PYGOSCELID PENGUIN COLONIES IN THE ANTARCTIC

Using census data on three species of pygoscelid penguins, we tested t he hypothesis that competition for food during the chick-provisioning stage of reproduction limits the number of conspecific seabirds in a r egion. This prey-depletion hypothesis predicts that a negative correla tion should exist between colony population size and the total number of breeding pairs from other colonies within parental foraging range. We also tested whether or not a negative correlation exists between co lony size and the population size of, or distance to, the nearest neig hboring colony. Suitable data were available for Adelie, Chinstrap, an d Gentoo penguins (pygoscelis adeliae, P. antarctica, P. papua) along the coasts of Victoria Land and the Antarctic Peninsula, where major p ortions of these species' world populations nest. Results indicated th at colonies were highly clustered, with small colonies grouped around one or two large ones, in turn spaced widely. Depending on species, tw o different patterns of geographic structuring were observed. For the Adelie and Gentoo penguins, no significant negative correlation existe d between colony size and the total number of pairs breeding within pa rental foraging range of the reference colony; however, a significant negative correlation occurred at 150 and 200 km, well beyond foraging range. We found no relationship between colony size and size of or dis tance to the nearest neighboring colony. In contrast, for the Chinstra p Penguin, a significant positive correlation existed between colony s ize and total breeding population within the foraging range (50 km) bu t, as with the other two species, the correlations became more negativ e at greater distances. Moreover, a significant positive correlation e xisted between colony size and size of, but not distance to, the neare st colony in this species. We confirmed the hypothesis previously put forward that prey depletion by parents feeding chides cannot explain s ize structuring of seabird colonies where breeding-season food supply is superabundant, as in polar regions. However, we also showed that pr ey depletion is not a necessary condition for negative size structurin g. We suggest that if prey depletion occurs (by exploitative or interf erence competition), any manifestation in terms of colony distribution is overridden near to the colony by aggregating factors that original ly led penguins to be colonial and philopatric, for example, social fa cilitation or predator avoidance. We further propose that geographic s tructuring is better explained by factors affecting the metapopulation (all breeders and nonbreeders associated with the colony cluster), es pecially during the prebreeding period, than by factors affecting chic k-provisioning parents alone.