1. Departamento de Zoología; Facultad de Ciencias; Universidad de Granada, E-18071; Granada Spain
2. Behavioural and Physiological Ecology Group; Centre for Ecological and Evolutionary Studies; University of Groningen; 9700 CC Groningen The Netherlands
3. Department of Wetland Ecology; Estación Biológica de Donaña; CSIC; Seville Spain