A migratory bird built for more than migration
The European nightjar makes an extraordinary annual journey between Europe and southern Africa, but migration is only one part of its aerial life. At night, the bird hunts insects in flight, a job that demands a very different set of abilities: slowing down, turning sharply and maintaining control at low speeds.
New research from Lund University shows that those competing demands are visible in the nightjar’s flight mechanics. The study, published in PLOS Biology, describes wings that appear to be a compromise rather than a single-minded solution for efficient long-distance travel.
That conclusion challenges a familiar assumption about migratory birds. It can be tempting to picture a long-distance migrant as an animal optimized above all for economical sustained flight. The nightjar, however, must also be an agile nocturnal hunter. Its wings have to serve both roles.
Testing flight in a wind tunnel
Researchers studied nightjars flying at different speeds in a wind tunnel and measured the airflow behind them. This allowed the team to examine how the birds generated lift and thrust across their flight range rather than inferring performance from wing shape alone.
The results point to a cost associated with the bird’s broad wingtips. Those wingtips are likely useful when the bird is flying slowly, which matters during insect hunting and maneuvering. But as speed rises, the study found that lift production becomes less efficient.
Christoffer Johansson, an associate professor at Lund University, described the result as evidence of competing aerodynamic demands. The nightjar can fly slowly and maneuver when hunting, but that capability is paired with reduced efficiency in lift generation at higher speeds.
The important point is not that the bird is poorly adapted. Rather, the study suggests that adaptation is distributed across several needs. A wing can be successful because it works adequately, or even exceptionally, in multiple jobs that pull its design in different directions.
Why slow flight matters
For a bird that catches insects on the wing at night, low-speed handling is not an incidental feature. The animal has to remain airborne while responding to prey and changing direction. Broad wingtips can help support that kind of flight, even if the same configuration does not produce the most efficient lift performance as speed increases.
Migration adds another demand. The European nightjar travels vast distances between its breeding and non-breeding areas, meaning its body and wings must also support sustained travel. The new work frames the apparent tension between those functions as a biological trade-off.
Such trade-offs are central to evolution. An animal is not designed from a blank sheet for one setting; it carries the consequences of all the tasks that affect survival and reproduction. In the nightjar’s case, the need to hunt slowly and maneuver precisely appears to be reflected in the way its wings perform when the bird flies faster.
The findings also make clear why a single measure, such as migration distance, cannot fully explain the form of a bird’s wings. Flight performance depends on what the animal does throughout its life, including the speeds it uses and the maneuvers it must make while feeding.
An active upstroke
The researchers made a second, unexpected observation: the nightjar generates thrust while its wings are moving upward. In many descriptions of bird flight, the downstroke receives most of the attention because it is associated with generating force. The Lund study indicates that the upstroke can make an active contribution to propulsion in this bird.
Active upstroke thrust has previously been associated mainly with bats and insects rather than birds. Johansson said the result suggests that similar aerodynamic solutions can emerge in very different animals.
That convergence matters because bats, insects and birds do not share the same wing structures. Yet their flight can still be shaped by comparable physical constraints. The research suggests that an upstroke need not be only passive, or devoted only to lift generation. It can be used to produce thrust when the wings cannot be folded very much.
For the nightjar, that may be especially relevant given the bird’s wing form and its need to perform varied flight tasks. The study does not reduce the animal’s motion to a simple power stroke followed by a recovery stroke. Instead, it identifies a more continuous contribution from the wingbeat cycle.
A lesson for flight technology
The work has implications beyond bird biology. Engineers developing flying robots often face the same broad problem: a vehicle may need to travel efficiently while also hovering, slowing, turning or operating in confined spaces. Designs that excel at one task can lose performance in another.
The nightjar offers a natural example of a system that operates within those constraints. Its wings do not represent an idealized answer for only one speed or one mission. They reflect a working balance between slow, maneuverable hunting flight and long-distance movement.
That balance is the central discovery. The European nightjar’s migration remains remarkable, but its wings cannot be understood solely as equipment for a continental journey. They are also the tools of a hunter, and the aerodynamic costs of that versatility become visible as the bird flies faster.
By measuring airflow from live birds in controlled conditions, the Lund team has provided evidence that the compromise is mechanical as well as anatomical. The nightjar’s flight is not defined by a single optimum. It is defined by what the bird must be able to do.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
