1. Department of Migration and Immuno-ecology, Max Planck Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany
2. Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurstrasse 190, 8057 Zurich, Switzerland
3. Department of Collective Behaviour, Max Planck Institute for Ornithology, Universitätsstrasse 10, 78464 Konstanz, Germany
4. Chair of Biodiversity and Collective Behaviour, Department of Biology, University of Konstanz, Universitätsstrasse 10, 78464 Konstanz, Germany
5. Department of Anthropology, University of California Davis, 1 Shields Ave, Davis, CA 95616, USA
6. Smithsonian Tropical Research Institute, Luis Clement Avenue, Building 401 Tupper, Balboa Ancon, Panama
7. Edward Grey Institute of Field Ornithology, Department of Zoology, University of Oxford, Oxford OX1 3PS, UK