Bird sampling was performed with permissions from landowners and the regional Department of the Environment (Comunidad de Madrid; Junta de Andalucía; Junta de Extremadura) and all the experimental procedures were approved by The Doñana Biological Station Ethics Committee on Animal Experimentation (CEEA-EBD) by CSIC Ethics Committee (CEC). Birds were captured in mistnets and retained until manipulation in cloth bags to keep them safe and calm. All sampled birds were released unharmed at the site of capture after manipulation.