1. Groningen Inst. for Evolutionary Life Sciences, Univ. of Groningen; PO Box 11103, NL-9700 CC Groningen the Netherlands
2. Dept of Animal Ecology; Netherlands Inst. of Ecology (NIOO-KNAW); PO Box 50, NL-6700 AB Wageningen the Netherlands
3. Vogeltrekstation, Dutch Centre for Avian Migration and Demography (NIOO-KNAW); PO Box 50, NL-6700 AB Wageningen the Netherlands