Common kestrel hovering toward wind

　The hovering of medium-sized birds may be realized by flying with the same speed as the wind against the wind. When the wind is weak, the lift must be taken precedence than the thrust. An increased drag may be permitted if it bring about an increase of lift. First, the kestrel enlarges the angle of attack and the curve of the wing, inclines down the opened tail, and makes the primaries of separated feathers. Farther, the kestrel attempts an increase of the lift by adding the back and forth motion and the alternate change of the angle of attack. If the wind calms down, the flapping down using the drag jointly and the back drive on the flapping up run will be necessary, though it is burdensome. The kestrel's much large tail is effective to the adjustment of the lift and drag in correspond to the change of wind. The flapping of the inclined wing up ward is necessary to keep the stability and to reduce the induced drag.
(Prod. Feb. 2006, full‐scale, No.1)