CONSEQUENCES OF WEIGHT DECREASE ON FLIGHT PERFORMANCE DURING MIGRATION

Migrating birds, flying nonstop over long distances, are substantially heavier at the start than at the end of their journey. Aerodynamic mo dels predict that these birds would optimally have to fly faster in th e beginning of their flight, and end at a slower speed. Energy expendi ture would be extremely high in the beginning, decreasing towards the end. Trained kestrels fly slower when carrying a load, generating the required extra Lift by changing the wingbeat kinematics. An allometric equation, describing the relationship between empirically derived fli ght costs at the maximum range speed and body mass, is used to calcula te the flight range of a wader that loses more than 60% of its lean bo dy weight during migration. Flight speed predictions are based on the kestrel data. Results of this novel approach are shown to provide more realistic predictions than those based on an aerodynamic model.