1. a NERC Environmental Systems Science Centre , The University of Reading , Berkshire, RG6 6AL, England, UK
2. b Ecology and Behaviour Group, Edward Grey Institute of Field Ornithology, Department of Zoology , The University of Oxford , Oxford, OX1 3PS, England, UK
3. c Royal Society for the Protection of Birds , The Lodge, Sandy, Bedfordshire, SG19 2DL, England, UK
4. d National Remote Sensing Centre Ltd , Delta House, Southwood Crescent, Farnborough, Hampshire, GU14 0NL, England, UK