Fig. 5. External photogrammetric measurement of the wing kinematics of a free-flying steppe eagle Aquila nipalensis coming in to perch on its handler's arm. Left panel shows one of a stereo pair of images taken at 500 frames s^–1. Right panel shows a calibrated reconstruction of the lower surface of the left wing based on stereo-matching of natural features of the plumage. Black points on the wing denote measurements, connected by straight lines to assist in visualizing the wing topography; the colour map denotes the local geometric angle of attack of the interpolated wing surface with respect to the horizontal. The isolated black points denote reference measurements on the head and tail, indicating the longitudinal axis of the bird. Note that whereas the angle of attack and camber of the proximal section of the wing is relatively consistent in a spanwise direction, the distal portion of the wing is set at a much greater angle of attack. This reflects the angle of attack of the interpolated surface and does not take account of the local twist of the primary feathers, which will be measured in future work. An animation of this perching sequence is available (Movie 1 in the supplementary material).