1. Max Planck Institute for Ornithology, Department of Behavioural Neurobiology, D-82319 Seewiesen, Germany
2. Present address: Department of Zoology & Entomology, University of Pretoria, Hatfield 0028, South Africa
3. Present address: Pädagogische Hochschule Oberösterreich, A-4020 Linz, Austria