Most birds move forward when they push their wings down. When they lift their wings up, they usually fold them.

But scientists in Sweden found something new. They flew three nightjars, birds that hunt insects at night, in a wind tunnel. The birds made forward power when they lifted their wings, too. Nightjars have wide wings. These wings are good for slow flight, but not for fast flight.

In most birds, the downstroke of the wing produces the force that moves them forward, while the upstroke mainly helps support their weight. However, researchers at Lund University in Sweden have found that European nightjars break this rule. Nightjars migrate long distances between Africa and Europe, but they also fly slowly and hover while catching insects.

The team flew three nightjars in a wind tunnel and studied the air behind their wings. During normal flight, the outer part of the wing produced forward thrust during the upstroke, something mainly known in bats and hummingbirds. Their broad wings, however, made fast flight less efficient, showing a trade-off between hovering ability and speed.

Textbook accounts of bird flight hold that the downstroke generates thrust, while the upstroke, with the wing folded to reduce drag, mainly contributes to weight support. A study published in PLOS Biology by researchers at Lund University challenges this view. European nightjars are an ideal test case because they combine long-distance migration between Africa and Europe with slow, maneuverable flight and hovering when hunting insects, activities that favor different wing shapes. The team selected three wild nightjars able to fly at a range of speeds, flew them in a wind tunnel, visualized the wake behind their wings and analyzed wing motion in three dimensions before releasing the birds.

At cruising speeds, the researchers observed an unusual pair of vortices at each wingtip, associated with reduced aerodynamic efficiency; the broad, slotted wingtips seem to aid slow flight but carry a cost at higher speeds. More surprisingly, the outer wing produced thrust during the upstroke, a feature previously attributed mainly to bats and hummingbirds. Because the birds could not fold their wings much during the upstroke, the authors suggest this restriction may underlie the extra thrust. With only three birds tested in an artificial setting, however, the findings need confirmation.