1. Department of Collective Behaviour, Max Planck Institute for Ornithology; Universitätsstrasse 10; 78457 Konstanz Germany
2. Chair of Biodiversity and Collective Behaviour, Department of Biology; University of Konstanz; Universitätsstrasse 10, 78457 Konstanz Germany
3. Department of Biology & Redpath Museum; McGill University; 1205 Dr Penfield Avenue, Montreal, Quebec H3A 1B1 Canada
4. Department of Behavioural Ecology and Evolutionary Genetics; Max Planck Institute for Ornithology; 82319 Seewiesen Germany
5. Edward Grey Institute of Field Ornithology, Department of Zoology; University of Oxford; South Parks Road, Oxford OX1 3PS U.K.