Fig. 1. Three selected tracks of swifts in nocturnal roosting flight. The blue line shows the ground track, and the red lines represent the heading direction every 10s. Numbers denote the time (in s) after the start of tracking. The arrows beside the letters indicate the wind direction. Geographic north is upwards. Track A is a swift flying into a wind with a mean direction of 161° and a speed of 6.9 m s^-1. The index of straightness relative to the ground, IS_g, is 0.67. In track B, the swift is facing a wind of 8.1 m s^-1 at 314°, and IS_g is 0.59. The corresponding values for track C are 2.9 m s^-1, 190° and 0.22.