1. IICB, University of Leeds, Leeds, LS2 9JT, UK
2. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
3. Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark
4. Department of Biology, University of Pisa, Via A. Volta 6, I-56126 Pisa, Italy
5. Alterra, Center for Ecosystem Studies, PO Box 47, 6700 AA Wageningen, The Netherlands
6. New Jersey Audubon Society, Cape May Bird Observatory Centre for Research and Education, Cape May Court House, NJ 08210, USA
7. Max Planck Institute for Ornithology, Department for Migration and Immuno-ecology, Schlossallee 2, Radolfzell 78315, Germany