Fig. 5. Deviations in the Y direction (up/down) of points on the neck. The deviations are relative to the mean swimming direction of the birds (red broken arrow). The Y axis is the same as described for Fig. 4B, and the X axis is the same as described for Fig. 2. The points are the tip of the bill and the base of the neck. Several points are missing at the end of the cycle, as the birds exited the view of the camera. It is clear, however, that the tip of the bill moves differently from the body (represented by the base of the neck), ascending when the body descends during the stroke phase and vice versa during the end of the glide and the recovery phase.