Nesting Density and Reproductive Success in a Colonial Seabird, the Magellanic Penguin

A central tenet of density-dependent habitat selection theory, that individual fitness generally declines with increasing population density, has led to the hypothesis that, for colonial species, living at high density produces advantages that outweigh the costs. Seabirds have provided evidence for this hypothesis, and positive effects of high density have been offered as explanations for coloniality in this overwhelmingly colonial group. We investigated effects of nesting density on reproductive success of a colonial seabird that nests at variable densities, the Magellanic Penguin (Spheniscus magellanicus). Using breeding success data collected from 1984 to 1991 as part of a long-term study of this species at a large colony at Punta Tombo, Argentina, and experiments that included manipulation of nesting density, we addressed effects of nesting density at two spatial scales: the within-colony area scale (hundreds to thousands of square meters) and the very small scale of a penguin's immediate surroundings (distance to nearest neighbor). Nest density at the colony averaged 8.7 active nests/100 m2and ranged from 50 active nests/100 m2(n = 873 plots). In 100-m2plots sampled from 1987 to 1990 over large areas of the colony of similar average nest quality and distance inland, the number of fledglings per active nest was inversely correlated with nest density. Experienced breeders using high-quality nests in a high-density study area (>20 nests/100 m2) had 0.25 fewer fledglings per active nest than similar birds nesting in a low-density area (