Fig. 4. Inertial measurements from a steppe eagle in soaring flight. The graph plots total measured acceleration against time: all three components of acceleration, angular velocity and orientation are logged by the inertial measurement unit, but are not shown. The thumbnails show synchronized frames from a hand-held camcorder (upper row) to provide context, and from a rearward-facing onboard camera (lower row) to confirm that the instrumentation remains steady throughout. Dashed lines denote the correspondence of the graph with the numbered frames. Note how the circled tan-coloured rump contour feathers remain steady (position of circle identical between images), indicating that the instrumentation is static with respect to the body. The visible transients therefore denote real accelerations of the bird, and are presumably excited by gusts, etc., as the bird is not actively manoeuvring in this sequence. The downy white feathers that are visible on either side of the circled contour feather are blowing freely in the wind, so provide no information on the position of the instrumentation with respect to the body.