Movie S1.

This animation shows the position of the birds in the horizontal plane relative to the center of mass of the flock for a 43-min phase of the second migratory stage. The first 3 min of the flight, which the birds spent circling to gain height, were omitted. The frame captures an area of 100 × 100 m, with the center of mass of the flock in the center of the frame. The animation runs at 80× time lapse, and the arrow in the upper right corner indicates the average travel direction of the 14 birds.