Fig. 4. Representations of vortex wakes shed from the wings of a thrush nightingale Luscinia luscinia at slow (A), medium (B) and fast (C) flight speeds in a wind tunnel, measured using digital particle image velocimetry [DPIV (from Spedding et al., 2003)]. Red and blue indicate the wake from upstroke and downstroke, respectively. Both phases of the wingbeat are aerodynamically active at each speed, and there are prominent cross-stream vortices apparent at the ends of half-strokes during slower flight (A,B) and throughout the wingbeat cycle during faster flight (C).