1. Department of Animal Ecology; Netherlands Institute of Ecology; Wageningen 6708 PB The Netherlands
2. Institute of Biodiversity; Animal Health and Comparative Medicine; University of Glasgow; Glasgow G12 8QQ UK
3. Sensory Ecology Group; Max Planck Institute for Ornithology; Seewiesen 82319 Germany
4. Communication and Social Behaviour Group; Max Planck Institute for Ornithology; Seewiesen 82319 Germany
5. BirdLife Austria; Vienna 1070 Austria