1. School of Biological and Biomedical Sciences, University of Durham, South Road, Durham DH1 3LE, UK
2. 2NERC Centre for Hydrology and Ecology, Banchory Research Station, Hill of Brathens, Banchory, Aberdeenshire AB31 4BW, UK
3. Dipartimento di Etologia, Ecologia ed Evoluzione, Universitádi Pisa, Via Volta 6, I–56126 Pisa, Italy
4. 4Consiglio Nazionale delle Ricerche, Istituto di Elaborazione dell'Informazione, Via Alfieri 1, I–56100 Pisa, Italy
5. 5Ornithology Group, Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK