1. Zoological Society of London, Institute of Zoology, Regent's Park, London NW1 4RY, UK
2. Department of Migration and Immuno-Ecology, Max Planck Institute for Ornithology, Am Obstberg 1, Radolfzell 78315, Germany
3. Department of Biology, University of Konstanz, Konstanz 78464, Germany
4. Earth Science Division, NASA, Washington, DC, USA