Fig. 8. Roll orientation through a left turn. (A) Changes in roll angle through time during a single trial, measured at the wing roots (Roll angle) and at the tail (Tail inclination angle). Roll angle changes both within and among wingbeats, typically reading a local minimum at mid-downstroke as the wings pass through the horizontal plane of the body coordinate system. (B) The power spectrum of the roll angle reveals that changes are largely confined to two frequencies, the bird's wingbeat frequency and a lower frequency encompassing approximately 3 wingbeats. (C) The peak-to-peak amplitude of the changes in roll near wingbeat frequency were approximately 16° and declined as the bird neared the end of the turn. In contrast, the peak-to-peak amplitude of the lower frequency, approximated by the red line in A, was approximately 45°.