Fig. 3. Spanwise vorticity, _y(x,z) for a mid-wing slice taken at x=17c for the thrush nightingale in the Lund wind tunnel. The reference frame moves with the mean flow, as for flight from right to left through still air. A sequence of four frames from consecutive wing beats is collected to cover just over one wake wavelength (). + signs show patches positive vorticity shed at the beginning of the downstroke; there are two in this sequence. The equivalent patch of negative vorticity, noted by the – sign, is much more diffuse. From the same dataset (but a different sample) as Spedding et al. (Spedding et al., 2003b).