Fig. 2. Straightness indices in different wind speeds. (A) The straightness index relative to the ground, IS_g. 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, IS_a. Circles indicate that the swift has been circling, i.e. that the bird has crossed its own flight path relative to the ground (IS_g) and the air (IS_a).