Nightjars trade off cruising speed efficiency for hovering capabilities and link upstroke thrust production with...

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.