Flight in Flocks: Surfing in the wake of other birds?

Group flight in a V-formation is a means to improve energy efficiency. But is this also the case for flight in bird flocks or is flocking only beneficial for protection against predators and for foraging strategies? Here we quantify the metabolic and aerodynamic costs/benefit of bird flight in vortex wakes, using wind tunnel flight tests with European Starlings, known for flocking in huge numbers. Birds are tested in a wind tunnel, flying either solo or with one or more companion birds. We fly the birds either in a clean flow or in the wake of an actuated airfoil. The birds' responses are measured using (i) a camera system 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. We find that the starlings' mean relative locations in flights with a companion bird align as in V-formation flight with a lateral offset of around ½ wingspan. This suggests that they can benefit from the wingtip vortex of the leading bird.