Scissor-Tailed Nightjars Clap Their Wrists in the Dark to Court Mates, Researchers Find

Between 3 and 4 in the morning, near the full moon, a male scissor-tailed nightjar in northern Argentina fans its long forked tail like an open pair of scissors and, while trying to mate, snaps its wings together at the wrist. The result is a sharp clap. Ornithologists working in South America had heard such snapping around these birds for years without being able to say how it was made. A research team has now filmed it in detail for the first time.

The work is published in the Journal of Avian Biology. It was led by Juan Ignacio Areta of CONICET, Argentina's national research council, who is the paper's lead author, and co-led by Christopher Clark, a biologist at UC Riverside. The project began during a sabbatical Clark spent in Argentina. The two decided to focus on the scissor-tailed nightjar because it proved easier to observe than birds living deep in forests or dense vegetation. Areta helped pick suitable species and did extensive fieldwork alongside Clark.

Nightjars are related to hummingbirds, though they are nocturnal and look more like small owls. Mottled brown feathers camouflage them against rocks and dirt, and large eyes help them hunt moths and beetles in the dark. The males are the showier sex, with the forked tails that open during courtship.

To watch without disturbing them, the team used high-speed infrared cameras during the predawn hours, when noise was low enough to observe the birds undisturbed. Infrared light is invisible to the birds. "You have to approach the birds on their own terms," Clark said, noting that the light let the researchers watch without affecting behavior.

The footage settled the question: the sound is not vocal. The males physically collide the radius bones of their wings, the bird counterpart of the human forearm, producing the clap during courtship and copulation.

The team also checked museum specimens for specialized wrist structures that might make the snap easier or louder. They found no obvious anatomical modifications. Clark offered a comparison: humans aren't specially adapted to clapping their hands, yet can make a loud sound, and the birds may likewise need no major structural change.

Non-vocal displays have been documented in a handful of other birds, including tropical manakins, but the mechanics behind them remain poorly understood. Clark said many species make important sounds with wings, feathers, claws and bones rather than voices, and described them as a hidden corner of biodiversity. His lab studies the physics of unusual animal sounds, including whether some wing-generated noises create tiny shock waves like those formed when metal objects collide at high speed.

A larger question is what such sounds can say. Human speech carries enormous variation and subtlety, and researchers don't yet know whether wing snaps can convey comparably nuanced messages. Clark asked whether the birds can evolve different kinds of snaps or are limited to one basic signal.

The team also recorded a second unusual sound during aerial chases between birds, and its source is still unknown. "We still have some questions, but we're getting closer to understanding the secret language of these birds," Clark said. His longer-term hope is a complete list of the strange ways birds make sound.