Density dependence is considered a key factor limiting colony size and reproductive success in colonial seabirds. Intraspecific competition at large colonies should force birds to travel farther to find food, which is thought to limit colony size by reducing reproductive success. We used measurements of maximum foraging distance from 17 colonies to examine the relationship between colony size and foraging range in thick-billed murres (Uria lomvia). Foraging range is expected to increase with the square root of colony size; this prediction was supported by our tracking data, which showed that foraging range scaled to the 0.44 power of colony size. We developed a bioenergetics model to examine how colony size effects chick provisioning, fledging mass, and reproductive success. The model identified thresholds where reproductive success declines sharply with colony size and prey quality. Reproductive success at smaller colonies should be resilient to changes prey quality and prey availability, while large colonies depend on access to high quality prey in order to fledge chicks. Colony size plays an important role in determining foraging behaviour and reproductive success of colonial breeding species. Our approach demonstrates how behavioural theory can be used to inform conservation and management of wildlife; in this case, to identify marine foraging habitat around colonies and estimate prey requirements for large colonies.

Past NRESi colloquium presentations and special lectures can be viewed on our video archive, available here.