Level 1 - Absolute Beginner

The nightjar is a bird. It flies at night. It eats insects.

Every year it flies from Europe to Africa. Then it flies back. This is a very long way.

Scientists in Sweden study the bird. They put it in a wind tunnel. A wind tunnel blows air, so the bird can fly in one place.

The wings have wide tips. Wide tips are good for slow flying. But they are not good for fast flying. This is a surprise.

bird

An animal with feathers, two wings and a beak.

night

The dark time when the sun is down.

insect

A very small animal with six legs, such as a fly or a bee.

fly

To move through the air.

wing

The part of a bird that it uses to fly.

wide

Big from one side to the other side.

slow

Not fast.

surprise

Something you do not expect.

Level 2 - Elementary

The European nightjar is a night-flying bird that hunts insects in the air. Each year it migrates between Europe and southern Africa, a journey of thousands of kilometres. You might expect such a traveller to be built for efficient long flights.

Researchers at Lund University in Sweden tested that idea. They let nightjars fly in a wind tunnel at different speeds and measured the air moving behind them. The study was published in the journal PLOS Biology on October 9.

The results were not what the team expected. The bird's wing tips are broad, which helps it fly slowly and change direction quickly while catching insects. But as the bird speeds up, those same broad tips make its wings less efficient at producing lift.

Christoffer Johansson, who led the work, said the finding runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine. In other words, the bird is a compromise rather than a perfect design.

migrate

To travel from one region to another at the same time each year.

hunt

To chase and catch animals for food.

wind tunnel

A machine that blows a steady stream of air so flight can be studied.

efficient

Working well without wasting energy.

lift

The upward force that keeps a bird or aircraft in the air.

broad

Wide from side to side.

journal

A magazine in which scientists publish their research.

compromise

A result that balances two things you cannot both have fully.

Level 3 - Intermediate

The European nightjar, Caprimulgus europaeus, is a nocturnal insect hunter that migrates each year between Europe and southern Africa, covering thousands of kilometres in each direction. Evolutionary logic suggests that such a commuter should carry wings tuned for economical cruising. A study published in PLOS Biology on October 9 by Christoffer Johansson of Lund University suggests the opposite.

Johansson's team flew nightjars in a wind tunnel across a range of speeds and measured the airflow trailing behind them, which reveals how much useful force a wing is generating and at what cost. The broad tips of the nightjar's wings, they found, probably assist slow flight and manoeuvring, the very skills required to intercept insects in darkness. The same geometry becomes a liability at higher speeds, reducing the efficiency with which the wing produces lift.

The second finding is the more unexpected one. The birds generate thrust during the upstroke, the part of the wingbeat usually treated as a recovery phase in which the wing is simply repositioned. Thrust on the upstroke has been documented in bats and insects, but it has not generally been attributed to birds. Johansson links it to the nightjar's restricted ability to fold its wings: if a wing cannot be tucked away on the way up, it may as well be made to do useful work.

The implication is a reframing rather than a demotion. Johansson observed that the result runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine. Natural selection has not produced an ideal cruiser that happens to hunt; it has produced an animal that pays a measurable penalty on migration in order to be lethal at low speed. The team suggests the upstroke mechanism may also inform the design of flapping drones and flying robots.

nocturnal

Active at night rather than during the day.

manoeuvring

Moving and turning with skill and control.

geometry

The shape and proportions of something.

liability

A disadvantage, something that causes a problem.

thrust

The forward force that pushes a bird or aircraft ahead.

upstroke

The upward half of a wingbeat.

intercept

To catch or stop something while it is moving.

natural selection

The process by which traits that help survival become more common over generations.

Level 4 - Advanced

Among the tidier assumptions of flight biomechanics is that long-distance migrants should be optimised for cruising. An animal that crosses continents twice a year pays an enormous energetic bill for locomotion, and selection ought to have shaved that bill wherever it could. The European nightjar, Caprimulgus europaeus, declines to cooperate. In work published in PLOS Biology on October 9, Christoffer Johansson of Lund University flew nightjars in a wind tunnel across a span of speeds, read the wake they left behind, and found an animal that is conspicuously not built for the journey it makes between Europe and southern Africa.

The culprit is the wing tip. Nightjar wing tips are broad, a geometry that plausibly serves slow flight and tight manoeuvring, which is precisely what an aerial insectivore operating in darkness requires. The cost arrives with velocity: as speed rises, that same breadth degrades the efficiency with which the wing converts effort into lift. The bird is therefore carrying a hunting instrument on a migratory route, and absorbing the penalty twice annually. Johansson's own gloss is the useful one, that the result runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine.

The study's more consequential claim concerns the upstroke. Conventional accounts of avian flapping treat the upstroke as largely passive, a repositioning of the wing between the downstrokes that do the aerodynamic work. Johansson's wake measurements indicate that nightjars extract genuine thrust from it, a capability previously associated chiefly with bats and insects. He connects this to a morphological constraint: the nightjar's wing cannot be folded far during the upward phase, and a wing that cannot be furled is better employed generating force than merely enduring drag. Constraint, in this reading, has been converted into function.

Two conclusions follow, one biological and one engineering. Biologically, the case argues against reading any animal as a solution to a single problem; the nightjar is an instrument of compromise whose migratory inefficiency is the price of nocturnal predation, and a reminder that selection optimises fitness rather than any particular performance metric an observer finds legible. Practically, a mechanism that produces thrust from a wing which cannot fold is of obvious interest to designers of flapping-wing drones, who face the same constraint by construction and have generally treated the upstroke as dead time to be minimised rather than exploited.

biomechanics

The study of movement and mechanical function in living bodies.

locomotion

The act or means of moving from place to place.

wake

The trail of disturbed air or water left behind a moving body.

insectivore

An animal that feeds on insects.

degrade

To reduce the quality or effectiveness of something.