Nightjar wings are built for compromise
Wind-tunnel experiments show how a design suited to slow aerial hunting sacrifices some efficiency during faster migratory flight.
European Nightjars appear to pay an aerodynamic price for wings that must handle two very different jobs: slow, agile insect hunting and long-distance migration between Europe and southern Africa. Wind-tunnel research from Lund University shows that the same broad wingtips that help at low speed reduce lift efficiency during faster flight.
Nightjars pursue flying insects in darkness, often slowing sharply, turning and briefly hovering. They also travel thousands of kilometres on migration. The study asks how one wing design accommodates those contrasting demands rather than assuming that a long-distance migrant must be optimised for economical cruising.
The researchers trained five wild-caught European Nightjars to fly in a recirculating wind tunnel. Three birds produced sufficiently steady flights for detailed measurements across speeds from 2.6 to 11 metres per second. Infrared high-speed cameras recorded wing movement while laser-based particle imaging mapped the wake left behind each bird.
Their PLOS Biology paper, Nightjars trade off cruising speed efficiency for hovering capabilities and link upstroke thrust production with restricted wing folding, reports a distinctive double tip-vortex pattern at cruising speeds.
The nightjar's wings appear to be a compromise between several different demands.
Associate Professor Christoffer Johansson, Lund University
The broad, slotted tips support the bird effectively during slow flight because they increase the useful area of the fastest-moving outer wing. At higher speeds, however, airflow and lift were distributed less efficiently across the span, increasing the cost of generating vertical force.
That does not mean migration is beyond the birds' capabilities. It shows that selection has balanced several requirements rather than maximising one. A narrower or more pointed wing might improve fast-flight efficiency but make it harder to hover, manoeuvre and take insects at low speed.
The experiments also found that Nightjars generated forward thrust during the upstroke. Birds are often described as folding the wing and using the upstroke mainly to reset it, while strong active upstroke thrust is more readily associated with bats and insects.
Nightjars could not fold their broad outer wings completely during the upstroke. The resulting reversed vortex loops indicated that this movement was contributing thrust. The authors suggest that comparable mechanics may occur in other birds or flying animals whose wings cannot be tightly folded.
The sample was necessarily small and came from one Swedish study population. The measurements reveal how these individuals produced force under controlled conditions; they do not quantify the energy used over an entire migration or show how weather and behaviour change the balance in the wild.
The birds were released after the experiments. The team now wants to test how widespread active upstroke thrust is and whether the same trade-off appears in other species, work that could also inform flapping-wing aircraft designed to operate well at more than one speed.
10 Oct 2026
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