AI Insight
Researchers studied European nightjars flying in wind tunnels and discovered that their broad, slotted wing tips create an unusual double tip vortex pattern during cruising flight, which reduces aerodynamic efficiency at faster speeds but enables hovering capabilities needed for feeding. The study also revealed that nightjars generate thrust during the upstroke of their wing beat at cruising speeds, a feature previously thought to exist only in bats and hummingbirds, likely due to limitations in how much they can fold their wings. These findings demonstrate the aerodynamic tradeoffs birds make when they must perform multiple flight behaviors with conflicting efficiency requirements.
Why it matters
This research advances understanding of how wing design involves compromises between different flight modes, which could inform the development of more versatile aerial vehicles capable of both efficient cruising and slow maneuvering. The discovery of upstroke thrust generation in birds challenges existing assumptions about avian flight mechanics and may have broader applications in bioinspired engineering.
Understand the Science
by L. Christoffer Johansson, Gabriel Norevik, Sonja Friman, Anders Hedenström
The European nightjar (Caprimulgus europaeus) is a long-distance migrant that also uses slow, hovering, flight as part of its feeding ecology. This reflects complex tradeoffs in aerodynamic mechanisms and makes studies of their flight performance, in relation to their wing shape, particularly interesting. Here we reveal, using wake visualization and aerodynamic measurements of nightjars flying in a wind tunnel, a unique double tip vortex phenomenon at cruising speeds. This correlates with reduced span efficiency at these flight speeds, suggesting relatively high costs of generating lift. Our interpretation is that the nightjar’s broad, slotted wing tip shape promotes lift during slow flight while incurring reduced aerodynamic efficiency at faster speeds. In addition, we found outer wing upstroke thrust generation during cruising flight—a characteristic previously attributed to bats and hummingbirds—in nightjars. This challenges an existing paradigm that considers an active upstroke to primarily add weight support in birds. Our kinematic analysis suggests this phenomenon may stem from constraints on wing folding during the upstroke, something that may apply generally to flapping flight. Our results highlight the tradeoffs seen in animals engaged in multiple costly behaviors with different aerodynamics optimization criteria based on ecological demands and offer potential solutions applicable in bioinspired engineering.