1. Dept of Biology; Lund Univ., Ecology Building; SE-22362 Lund Sweden
2. Lake Ånnsjön Bird Observatory; Handöl 563 SE-83015 Duved Sweden
3. Dept of Ecology and Genetics, Evolutionary Biology Centre; Uppsala Univ.; Norbyvägen 18 D SE-75236 Uppsala Sweden
4. British Antarctic Survey, Natural Environment Research Council; High Cross, Madingley Road Cambridge CB3 0ET UK
5. Conservation Ecology Group, GELIFES, Univ. of Groningen; the Netherlands