Nightjars Generate Thrust on Upstroke in Wind-Tunnel Tests at Lund

The team flew five nightjars at speeds from 4 to 10 m/s inside a low-turbulence wind tunnel and quantified wake vortices with high-speed cameras. Broad wing tips that aid slow, turning flight during insect hawking created early flow separation above 7 m/s, lowering the lift-to-drag ratio compared with slender-wing migrants. This trade-off shows that selection for dual-use wings favors compromise over single-purpose optimization.

The active upstroke, previously documented mainly in bats and insects, supplies measurable thrust because nightjars cannot fold their wings as tightly as passerines. Force estimates indicate the upstroke contributes 15-20 percent of total thrust at cruising speed, a result that aligns with earlier kinematic studies of swifts but contradicts textbook models assuming passive recovery strokes in birds.

The findings imply that flapping-wing drones facing similar span constraints could adopt nightjar-style kinematics rather than strict biomimicry of efficient migrants. Future field recordings of free-ranging birds will test whether the pattern holds across other Caprimulgus species or remains a specialized solution to nocturnal aerial foraging.