Figure 3.

Velocity vector field superimposed on colour-coded vorticity field of key phases of the wingbeat for each speed. (a,f,k) 5.7 m s⁻¹, (b,g,l) 7.7 m s⁻¹ and (c,h,m) 9.9 m s⁻¹. (i) shows beginning of downstroke, (ii) shows mid-downstroke, (iii) shows end of downstroke, (iv) shows mid-upstroke and (v) shows end of upstroke. Bird silhouettes symbolize the different phases of the wingbeat. Distance from the swift to the image plane was between 0.3 and 0.4 m, which corresponds to 8–11 chord lengths. The colour map scales from −350 (blue) to +350 s⁻¹ (red).