Fig. 10. Three-dimensional wake structure of the swift in cruising flight. The image frame is as if the bird flew obliquely from right to left and slightly into the paper, leaving the trace of one wingbeat in still air, starting at the upper turning point. Green tubes show the wingtip vortices, cylinders in shades of red are spanwise vortices with positive circulation and cylinders in shades of blue have negative circulation. The colour intensities and tube diameters are proportional to the strengths of each component. The geometry is deduced from a combination of wingbeat kinematics and streamwise plane section data.