Bewick's Swans refuelling on pondweed tubers in the Dvina Bay (White Sea) during their spring migration: first come, first served

Current models of bird migration consider the optimal decision making of individuals irrespective of the behaviour of conspecifics. We observed the foraging behaviour of Bewick's Swans Cygnus columbianus during their stop-over in the Dvina Bay in the White Sea in spring 1995 and 1996. Here the swans could only feed on their main food, tubers of fennel pondweed Potamogeton pectinatus. during low tide. In the course of the staging period, the swans tended to forage at progressively lower water levels, indicating that they depleted their main food and exploited increasingly deeper parts of the tuber-bank. This reduced the swans' main foraging period from 6.0 h/tide on 20 May to 3.3 h/tide on 28 May 1996. We calculated that this must have greatly decreased the rate of refuelling during the staging period. In accordance with this, swans arriving early stayed shorter than those arriving late. It seems of paramount importance to the swans to arrive as early as possible at the stop-over site. We conclude that when food depletion occurs, a game theoretical approach is needed to improve our understanding of the birds' adaptive behaviour during their migration.