Fig. 8. A reconstruction from three consecutive frames of Fig. 6 to show the vortex wake over slightly more than one wingbeat cycle. The wake is shown as if left in still air by the bird passing from right to left. The silhouette is drawn approximately to scale and in the correct vertical (z) position but its horizontal (x) location should in fact be displaced by about 3 to the left (upstream) because the measuring station is that far downstream of the bird in the test section. During the time required for the wake to advect past the cameras (approx. 3T, or 0.21 s), the wake has moved downwards under its self-induced convection speed. The three component frames are matched approximately but the data are not edited or reinterpolated to improve the fit, and the borders are left outlined so their location is clear. The wingspan bar (2b) is placed to begin at the start of the downstroke. The wake wavelength is determined by the flight speed and wingbeat period and is shown as a double-arrowed bar. The relative time spent on downstroke and upstroke is given by the downstroke ratio, and can be verified from the wake picture. The colour bar and its scaling are as given in Fig. 6, and are fixed for all low-speed wake images (Figs 6, 8, 9, 11).