A species of hummingbird makes a chirping noise with its tail feathers, not its throat, a study using high-speed video has suggested.
The exact source of the noise from male Anna's hummingbirds has been the subject of debate among researchers.
By using specialised footage, a team of US scientists were able to show that male hummingbirds' tail feathers vibrated during high-speed dives.
The findings appear in the Proceedings of the Royal Society B journal.
The loud chirp sound is produced by male Anna's hummingbirds (Calypte anna) as the birds dive towards the ground at speeds that exceed 50mph (80km/h) during their displays for nearby females.
Hitting the right note
The researchers, Chris Clarke and Teresa Feo from the University of California, Berkeley, wrote in their paper that they had gathered evidence that put an end to the uncertainty surrounding the source of the sound.
"Production of the sound was originally attributed to the tail, but a more recent study argued that the sound was vocal.
"We use high-speed video of diving birds, experimental manipulation on wild birds and laboratory experiments on individual feathers to show that the dive sound is made by tail feathers," they explained.
The pair added that while bird vocalisation had received considerable attention, non-vocal or "mechanical" sounds had been "poorly described".
"A diverse array of birds apparently make mechanical sounds with their feathers. Few studies have established that these sounds are non-vocal, and the mechanics of how these sounds are produced remain poorly studied," the scientists wrote.
They said the footage revealed that the trailing vane of the birds' out-tail feathers fluttered rapidly to produce the sound.
The characteristic dive caused a subtle change in the shape of these feathers to produce a loud chirping sound, the researchers noted.
"Many kinds of birds are reported to create aerodynamic sounds with their wings or tail, and this model may explain a wide diversity of non-vocal sounds produced by birds," they concluded.