Figure 2.

Schematic sketches to illustrate the relative positions of the light sheet and the bird. The light sheet defines a vertical plane with respect to the bird's body; this plane intersects the bird's tail. The velocity data in the region of interest (a square) were calculated to be the average speed of the flow above the tail. The length of the square (typical range from 2 to 3 cm) was chosen as the tail span that was intersected by the light sheet.