Foraging dynamics in goose flocks: the cost of living on the edge

The effects of flock position on the foraging performance of individual barnacle geese, Branta leucopsis, were determined by comparing foraging behaviour, vegetation quality and diet of geese at the edge and centre of feeding flocks. Birds on the edge of flocks fed in a hurried fashion compared with centre positions (shorter bouts of foraging) probably because of the increased amount of vigilant behaviour and aggressive encounters. In energetic terms, however, the costs of grazing and vigilance balanced out, so that it was as ‘expensive’ to feed in either flock position. Geese appeared to obtain more food when in edge positions because they pecked faster and obtained more food per peck as a result of the higher biomass of grass. A series of calculations, which included measures of energy expenditure, gross intake and energetic quality of the food, showed that the net energetic payoff of birds in edge positions was substantially higher than that of those in the centre. The relative merits of taking positions in the flock edge for single geese, pairs and families are discussed. The finding that dominant families were most often in edge positions and avoided the centre, and the fact that families attacked neighbours more often than did any other geese, suggests that they actively attempt to monopolize the best food resource available to flock members which is usually on the edges of the flock.