Great tits (Parus major) reduce body mass in response to wing area reduction:: a field experiment

Flight performance is crucial in determining whether a small bird will survive all attack by a predator. Given the importance of body mass in determining flight performance, it has been suggested that birds should strategically regulate body mass as a response to predation risk. However, all experiments tip to now have been carried out with captive birds, comparing experimental to control birds. Here we present data front the first experiment in the field using a within-individuals experimental design. The wing area of wild great tits, Parus major was reduced by reversibly taping primaries five to seven. This allowed for the same individual to alternatively act as control or experimental bird. Great tits reduced body mass (but not pectoral muscle width) during episodes of wing area reduction, lending support to the view that the reduction in body mass experienced by birds during molt is a strategy rather than the result of energetic stress. Theoretical models establishing the different trade-offs that determine optimal body mass should therefore take into account this important life-history episode.