1. Department of Migration and Immuno-Ecology, Max Planck Institute for Ornithology, Radolfzell, Germany.
2. Department of Biology, University of Konstanz, Konstanz, Germany.
3. Department of Collective Behaviour, Max Planck Institute for Ornithology, Konstanz, Germany.
4. MTA-ELTE Statistical and Biological Physics Research Group, Hungarian Academy of Sciences, Budapest, Hungary.