1. Ornithology Group, Graham Kerr Building, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK;
2. Scottish Universities Research and Reactor Centre (SURRC), East Kilbride, Glasgow G75 0QF, UK;
3. Centre for the Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 OHT, UK; and
4. Institute of Terrestrial Ecology, Banchory, Aberdeenshire AB31 4BY, UK