Surfing Birds: How birds interact with vortex wakes

Unsteady flow conditions - vortex wakes - play a significant role in the kinematics, dynamics and metabolic of animal flight. In this collaborative project we study how a medium-sized bird - European starling - reacts to an unsteady flow. A series of wind tunnel flight tests are performed in which the starlings are exposed to different wake structures generated by a wing located upstream. Different wakes are generated by flapping the wing (generating a reverse von Kármán Street, or thrust wake), or by setting the wing with a static angle of attack (generating either an upwash or downwash). A tip vortex can also be shed, if desired, by appropriate positioning of the wing tip in the test section. We measure the wake structure using PIV, and the bird response using (i) camera systems to record wing kinematics and preferred flight position, (ii) a lightweight inertial measurement unit (IMU) to record body motion, and (iii) the 13C-labelled sodium bicarbonate method (NaBi) to record the metabolic cost of flight. By combining kinematics, metabolic, and aerodynamic results, we formalize, and test hypothesized predictive relationships between wake structure, flight behavior and metabolic energy expenditure.