Fig. 1. These images, taken from a high-speed recording of a cockatiel flying at 1 m s^–1, show the tip-reversal upstroke. In the first frame, the wing has already reversed direction and the humerus has been elevated. In the second frame, the primary feathers have rotated slightly to create gaps between successive feathers. Between the second and third frames, the rotated primaries sweep upward as the wrist joint extends. By the third frame, the primaries have been rotated back into their standard orientation and the wing has begun to move forward as well as upward.