The wake of a jackdaw in slow forward flight is described. The three-dimensional velocity field was investigated qualitatively and quantitatively by analysis of multiple-flash stereophotographs of the motion of neutrally buoyant helium bubbles. The best description of the wake structure appears to be a chain of planar, nearcircular, discrete, small-cored, vortex loops, each produced by vorticity shed during a single downstroke.
However, the momentum measured in such a wake is approximately 35% of that required for weight support under these flight conditions. Some evidence is presented that this apparent wake momentum deficit may arise because the description of the real wake vorticity distribution is too simplistic.
The implications of these results for theoretical models of bird flight are briefly discussed.