Indoor flight experiments with trained kestrels. II. The effect of added weight on flapping flight kinematics

Two kestrels, a male and a female, were trained to fly over 50 and 125 m in a windless corridor. Both distances were flown with or without dead weights attached to the feet during 13 flight sessions for each bird. Added weight was either 0-3 N (31 g lead) or 0-6 N (61 g). Each session was devoted to one distance and one flight weight category. Flight duration was automatically recorded at the landing points and at four positions along the track. Gliding bouts were hand-clocked and flight altitudes were estimated with the aid of sidewall markings. An analysis was made of 1226 flights by the female over a total of 100 km, and 1017 flights by the male over 846 km. Different flight strategies were observed under the different experimental situations, and were compared with model predictions for optimal speeds. In the unloaded situations the birds flew at velocities close to the maximum range speed. Under load, speeds were lower and close to the predicted speeds for minimum power when 0-6 N was added to the weight