The nightjars’ torrid nights are punctuated by a deeply impactful male performance.
A series of sharp fissures divides the night air of a forest in the Andean foothills. But it’s not the sound of boots snapping twigs underfoot. It’s a bird.
Male scissor-tailed nightjars (Hydropsalis torquata) create these abrupt sounds by banging their wing bones together in a loud snapresearchers report in May Journal of Avian Biology.
Nightjars are primarily nocturnal, insectivorous birds related to hummingbirds and swifts. H. torquata males are unusual for their exceptionally long, paired tail feathers. These males were already known to emit explosive cracking noises at night as a mating signal to nearby females. But little was known about how they did it, says Juan Ignacio Areta, an evolutionary naturalist at the Instituto de Bio y Geociencias del Noroeste Argentino in Salta, Argentina.
“Many nocturnal animals are known to be extremely quiet, such as the ‘silent’ flight of owls,” says Areta. “We wanted to know how it was possible for a nocturnal animal to make such loud sounds.”
In late 2022, Areta and Christopher Clark – a behavioral ecologist at the University of California, Riverside – secretly filmed the male birds at night along a logging road near Salta. They used high-speed infrared cameras, then compared the images to the sounds they recorded.
Often, nightjars would jump off the ground and swing their wings behind their backs, creating a loud snap on impact. Sometimes males did this by stealing or mating with a female. Birds didn’t just bang their feathers together. It was clear to the researchers that the snaps were coming from the wrist bones colliding just below the wing’s final turn. Areta and Clark believe that the bones vibrating from the violent collision create a sudden clicking sound.
The results add scissor-tailed nightjars to a list of birds that use their bodies, not their voices, to make sound. Members of this drumming group include male Siberian grouse (Falcipennis falcipennis), which strike their uniquely shaped feathers together. Male riflemen scratch their beak on their wings like a wooden grater. Some manakins – colorful birds of the tropical Americas – are the only other birds known to clasp their wrists like nightjars, Areta says.
Researchers don’t know what these shots convey to other nightjars. Males snap when attracting, approaching and mating with females. They also snap when chasing intruders.
“It seems like nightjars are really fond of these sounds,” says Areta.