1. Wildfowl and Wetlands Trust; Slimbridge Gloucestershire GL2 7BT UK
2. Dept of Animal Ecology, Netherlands Inst. of Ecology; Droevendaalsesteeg 10 NL-6700 AB Wageningen the Netherlands
3. Computational Geo-Ecology, Dept of Science; Inst. for Biodiversity and Ecosystem Dynamics, Univ. of Amsterdam; PO Box 94248 NL-1090 GE Amsterdam the Netherlands