Fig. 2. Straightness indices in different wind speeds. (A) The straightness index
relative to the ground, ISg. A low value means that the
resulting flight path is greatly convoluted (see
Fig. 1 track C), while a value
close to 1 means that the swift has been flying on a straight course. (B) The
straightness index relative to the surrounding air, ISa.
Circles indicate that the swift has been circling, i.e. that the bird has
crossed its own flight path relative to the ground (ISg)
and the air (ISa).
