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

A male and a female kestrel were trained to fly over a mirror in a corridor, with lead weights attached to the feet to increase weight in the range 0-0-6 N (0-61 g). Films taken at 200 frames s"1 allowed three-dimensional analysis of the displacements of the head, tail and wingtips. A kinematic analysis, based on Fourier series, was made of the displacements in three perpendicular directions and of periodic changes of wing areas and tail areas. With increased mass, there was a decrease in flight speed, increase in wing beat frequency, increase in vertical wing beat amplitude, and an increase in the inclination of the tail. The shorter wing beat cycle under load was mainly due to a decreased upstroke duration, implying that the relative importance of the down-stroke was increased. The tail area and to a lesser extent the wing areas were enlarged during flights with the largest added weight