Evidence of intra-specific competition for food in a pelagic seabird

The factors affecting the population dynamics of seabirds have long intrigu ed biologists(1-5). Current data suggest that density-dependent depletion o f prey during the breeding season may regulate population size(6-9). Howeve r, much of the evidence for this has been circumstantial, and the underlyin g mechanisms are unclear(5,10). Here, we show that the per capita populatio n growth rates of northern gannet Morus bassanus at colonies in Britain and Ireland have declined with increasing population size. Furthermore, direct observations reveal that the mean foraging trip duration of breeding ganne ts is positively correlated with colony size, both among colonies of differ ent sizes in the same year, and within colonies as they change in size. To understand this phenomenon, we have developed a model which demonstrates th at disturbance of fish alone can readily generate conditions under which ga nnets at larger colonies have to travel further to obtain food.