High‐speed infrared video analysis of sonations in the scissor‐tailed nightjar <i>Hydropsalis torquata</i>

Many species of nightjar reportedly produce short, impulsive wing sounds during courtship, but the kinematics and physical mechanisms of sound production remain speculative. Using synchronized infrared high‐speed video and audio recordings we describe the mechanism of sound production of a sonation in the nocturnal family Caprimulgidae; the wing‐snapping of male scissor‐tailed nightjars Hydropsalis torquata . This sound is a short, sharp, loud, ‘ tk ', produced singly in a jump display (jump snap), in a syncopated series during a flight display (flying snaps), and in a fast series during copulation (copulation snaps). To produce these ‘ tk ' sounds, males rapidly elevated and supinated the wings to slam opposing wrists together. The videos falsify the hypotheses that sound is produced by intra‐ or inter‐wing colliding feathers or by clapping (i.e. a pulse of air accelerating to escape a constricted space, as in human hand‐clapping), as there was no contact by the surface of opposing wing‐feathers. Instead, the physical acoustic mechanism appears to be impulsive collisions between the wing bones (radii) which then vibrate, like wing‐snapping of Manacus manakins. A low‐frequency mechanical thud , produced by an unknown intra‐wing mechanism during the downstroke is also present in flying snaps and copulation snaps , and occurs independently during fast takeoff. Fluffle was produced by feathers rustling and colliding in preen‐like behavior that has likely a communicative function. An additional male display, the wing rattle , made in flight during chases, has a complex acoustic structure including a similar yet distinct mechanical impulsive sound, the thud , a whoosh, and vocal sounds, indicating a suite of possible sound production mechanisms. The wings seemingly do not touch during the wing‐rattle , suggesting that the second type of impulsive sound is produced by intra‐wing feather collisions. Preliminary comparisons indicate the conservation of homologous sound‐producing mechanisms and sounds in New World nightjars.