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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°.
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