1. School of Biological Sciences, Monash University, 25 Rainforest Walk, Clayton, Victoria 3800, Australia,
2. School of BioSciences, University of Melbourne, Melbourne, Parkville, Victoria 3010, Australia,
3. Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, D-78315 Radolfzell, Germany, and
4. Behavioural and Physiological Ecology, Groningen Institute for Evolutionary Life Sciences, University of Groningen, PO Box 11103, 9700CC Groningen, The Netherlands