The Nightjar’s Wing: A Compromise Written in Air

There is a quiet miracle in the way a bird moves through air, a negotiation between gravity and grace that most of us never pause to consider. We watch swallows dip and swifts soar, but rarely do we ask what it costs a creature to perform such ordinary magic. For the European nightjar, a bird that spends its days motionless on the ground and its twilight hours hawking insects in the dark, that cost has now been measured—and it reveals a compromise written into the very shape of its wings.

The nightjar is an unusual creature. It weighs about seventy grams, migrates thousands of miles between Europe and southern Africa each year, and yet it is also capable of hovering in place while hunting, a feat that demands vastly different aerodynamic tools . Its wings are long and narrow, with an aspect ratio of 7.5, which places it in the upper range for birds . Unlike most birds with such proportions, its wingtips are broad rather than tapered, a feature that would seem to work against efficient cruising flight.

Researchers at Lund University took the question into a wind tunnel, measuring the wake vortices left behind by nightjars flying at speeds ranging from just under three meters per second to eleven meters per second . What they found was a pair of aerodynamic surprises. At cruising speeds, the birds produced a double tip vortex—a phenomenon not normally seen in birds—which correlated with reduced span efficiency, meaning the wings were generating lift less effectively than their shape might suggest .

The second surprise was even more unexpected. During the upstroke, when most birds simply fold their wings and let the downstroke do the work of propulsion, the nightjar’s outer wing generated thrust . This is a trait previously associated with bats and hummingbirds, not with birds that rely on flapping flight . The kinematics suggest it stems from constraints on how much the wing can fold during the upstroke—a limitation that seems to turn a passive phase into an active one .

The trade-off is clear. The broad wingtips that help the nightjar hover and maneuver slowly during hunting cost it efficiency at higher speeds . “This runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine,” said Christoffer Johansson, the study’s lead author . Instead, the nightjar’s wings represent a balance between competing demands—slow, precise flight for feeding and efficient travel for migration.

The findings, published in PLOS Biology, offer more than a curiosity about one species. They suggest that the aerodynamic principles governing bird flight are more varied than previously understood, and that the upstroke may play a more active role than textbooks have allowed . Johansson noted that the same type of solution appearing in bats, insects, and now nightjars hints at a deeper principle: that the upstroke can generate thrust when wings cannot fold freely .

For engineers designing flapping drones and flying robots, the nightjar’s compromise may offer a model—not of perfection, but of adaptability. And for anyone who has paused to watch a bird wheel against a twilight sky, it is a reminder that every flight is a negotiation, and every wing a story of what was sacrificed for what was gained.

AI Image Disclaimer: The visuals accompanying this article were generated by artificial intelligence for illustrative purposes only and do not depict the actual research or birds studied.

Sources: PLOS Biology, Lund University, EurekAlert, EBSCO