Surfing on vortices: Bird flight responses to an unsteady vortex wake

We report on results from wind tunnel experiments examining the flight of European starlings (\emph{Sturnus vulgaris}) in both steady flows and in the presence of a structured vortex wake generated by a pitching airfoil. The investigation employs synchronized high-speed kinematic tracking alongside time-resolved Particle Image Velocimetry (PIV) to assess the birds' flight mechanics and interactions with unsteady flow. When flying in the presence of a vortex wake, the starlings exhibited distinct kinematic adjustments relative to their trajectories in clean steady flow. The wake footprints observed through PIV corroborated the kinematic measurements, revealing a phase alignment between the birds’ wingbeats and the periodic wake background.The combined circulation, resulting from the merging of the bird's flapping wake and the mechanically generated wake, closely resembles an in-phase superposition of the two components in the non-interactive cases.