Seabird colony distributions suggest competition for food supplies during the breeding season

Abstract

There is long-standing disagreement over the way in which seabird populations are regulated¹. They may be regulated by density-dependent mortality during winter as a consequence of food shortage^2–4, shortage of nest sites^5–7, social factors⁸, or not regulated at all⁹. Ashmole^10,11 argued that for tropical seabirds, density-dependent mortality of adults outside the breeding season is most unlikely because intraspecific competition for food close to the colony would result in prey depletion, causing adults to travel increasing distances for food as colony size increased.Breeding numbers could be regulated through a density-dependent reduction in reproductive output resulting from reduced rates of provisioning of chicks. This mechanism may not apply to seabird colonies in higher latitudes, however, because seasonal production might provide superabundant food supplies during the breeding season. Recent studies of the quantity of food consumed by breeding seabirds^12–15 suggest that prey depletion also occurs in temperate regions and invites a critical test of Ashmole's hypothesis. We present here an analysis of interactions between spatial distribution and size of seabird breeding colonies and provide support for Ashmole's suggestion that seabird numbers may be limited by intraspecific competition for food around colonies during the breeding season.