The nightjar migrates thousands of kilometres, yet the same wings must also help it catch insects at low speeds. A new study from Lund University shows how these competing demands have shaped the bird's flight performance.
The European nightjar (Caprimulgus europaeus) is a nocturnal migratory bird that travels between Europe and southern Africa each year. However, its wings are not only used for long-distance journeys. When hunting insects, the nightjar must be able to slow down, turn sharply and fly at low speeds. Researchers at Lund University have now shown how this trade-off is reflected in the bird's flight mechanics.
Using a wind tunnel, the researchers observed nightjars flying at different speeds and measured the airflow behind them. The results show that the bird's broad wing tips, which are likely advantageous for slow flight, come at a cost. As the bird increases speed, lift production becomes less efficient.
"This runs counter to the intuitive image of a long-distance migrant as a perfectly optimised flying machine. Instead, the nightjar's wings appear to be a compromise between several different demands. The bird can fly slowly and manoeuvre while hunting, but the trade-off is reduced efficiency in lift generation at higher speeds," says Christoffer Johansson, associate professor at Lund University.
The researchers also made another unexpected discovery. The nightjar generates thrust even when its wings move upward. Such an active upstroke has previously been associated mainly with bats and insects rather than birds.
"What's interesting is that the same type of aerodynamic solution seems to appear in quite different animals. This suggests that the upstroke does not have to be merely a passive or lift generating part of the wingbeat. It can be actively used to generate thrust when the wings cannot be folded very much," says Christoffer Johansson.
The findings provide broader insight into how evolution shapes animals that must perform several different tasks. A wing does not necessarily have to be optimised for a single type of flight. Instead, it may represent a balance between long-distance migration, foraging, slow flight and advanced manoeuvrability. The same principle may also be relevant for technology. Flapping drones and other flying robots, like birds, may be limited in how much their wings can fold during each wingbeat.
"We now want to investigate how common this way of generating force is among other birds and flying animals. It could help us understand which aerodynamic principles are specific to the nightjar and which are more general. This knowledge may also inspire the development of new types of flapping aircraft," Christoffer Johansson concludes.
