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

Citation
E. Spilling et al., Flock sizes in foraging White-fronted and Bean Geese in the Elbe valley and their effects on flight distance and time budget, J ORNITHOL, 140(3), 1999, pp. 325-334
Citations number
34
Language
TEDESCO
art.tipo
Article
Categorie Soggetti
Animal Sciences
Journal title
JOURNAL FUR ORNITHOLOGIE
ISSN journal
0021-8375 → ACNP
Volume
140
Issue
3
Year of publication
1999
Pages
325 - 334
Database
ISI
SICI code
0021-8375(199907)140:3<325:FSIFWA>2.0.ZU;2-8
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 t o 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 d ay. Feeding behaviour was sampled by scan sampling in 1995/96, and distance s of flight reactions to an approaching car were estimated in 1996/97 and 1 997/98. Usually, geese formed large flocks. More than 90% of individuals recorded s tayed in groups of more than 500 birds. In large flocks (several thousand g eese), prolonged feeding times at the cost of preening and resting behaviou r indicated a severe competition between individuals. Flight distances were lower in small flocks, but did not increase further with flocks becoming l arger than 150 birds. As shown in earlier studies, the benefit of flocking in terms of predator avoidance is unlikely to increase any further with gro ups exceeding a few hundred birds. One possible explanation of the observed flocking behaviour could be that m ost individuals in the population follow an opportunistic strategy when for aging. They join their foraging conspecifics instead of looking for feeding sites on their own. Flock size was limited by population size on the one h and, 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 wit h field size are likely to be the main factors determining and constraining flock size on agricultural fields.