Flock sizes in foraging White-fronted and Bean Geese in the Elbe valley and their effects on flight distance and time budget

Abstract

Flocking behaviour of foraging Whitefronted and Bean Geese (Anser albifrons, A. fabalis) was studied in the valley of the lower River Elbe from 1994 to 1998. Geese were counted every forthnight in the winter season of 1994/95 in a study area of 170 km(2), and daily in 1995/96 and 1996/97 in an area of 40 km(2). In the winter of 1997/98, counts were conducted every second day. Feeding behaviour was sampled by scan sampling in 1995/96, and distances of flight reactions to an approaching car were estimated in 1996/97 and 1997/98. Usually, geese formed large flocks. More than 90% of individuals recorded stayed in groups of more than 500 birds. In large flocks (several thousand geese), prolonged feeding times at the cost of preening and resting behaviour indicated a severe competition between individuals. Flight distances were lower in small flocks, but did not increase further with flocks becoming larger than 150 birds. As shown in earlier studies, the benefit of flocking in terms of predator avoidance is unlikely to increase any further with groups exceeding a few hundred birds. One possible explanation of the observed flocking behaviour could be that most individuals in the population follow an opportunistic strategy when foraging. They join their foraging conspecifics instead of looking for feeding sites on their own. Flock size was limited by population size on the one hand, and by field size on the other. Average maximum density of individuals in a flock was 0,1 birds per square meter. Thus, bird density together with field size are likely to be the main factors determining and constraining flock size on agricultural fields.