Fig. 3. Method used for identifying vortex cores and measuring circulation () in the wake of chukar Alectoris chukar as revealed using DPIV in a parasagittal plane at the mid-wing. In this instance, a juvenile chukar, no longer visible in the image, used WAIR to move upwards and toward the right through the illumination plane at an angle of 80°. Movements of its wings left vorticity () in the wake that was concentrated into two presumptive vortex loops, one per downstroke (D₁ and D₂), which were transected by the illumination plane. (A) Velocity vectors, expressed relative to average velocity, and with backround . (B) Streamlines associated with the vector field. Broken lines indicate regions sampled for (blue=starting vortex, red=ending vortex). (C) Above-threshold ^– (blue) and (D) above threshold ⁺ (red) was integrated with respect to area to measure contiguous circulation ( and ) or all circulation presumed to be associated with the core ( and ).