A small hummingbird has been found to "sing" through its tail feathers rather than its voice-box in the same way a wind musician plays a note on a clarinet.
Scientists have found that the Anna's hummingbird of the American south-west makes a "chirping" sound by dive-bombing at speeds of 50mph to cause wind to rush through its splayed tail feathers. The feathers quiver in the same way that the reed of a clarinet vibrates when a musician plays the instrument to produce a musical note. In this way, the bird is able to produce a noise that is louder than anything its own tiny voice-box can make.
Hear the Anna's hummingbird's 'chirp':
The researchers said it is the first time that any bird has been shown to make a deliberate noise in this way, but they now believe that there may be other species of hummingbirds that can sing through their feathers.
"This is a new mechanism for sound production in birds," said Christopher Clark, a doctoral student at the University of California, Berkeley, who was part of the study that revealed the hummingbird's secretive tail song.
"The Anna's hummingbird is the only hummingbird for which we know all the details, but there are a number of other species with similarly-shaped tail feathers that may use their tail morphology in producing sounds," said Mr Clark.
The scientists used high-speed cameras to record a male hummingbird's mating display as he dive-bombed a caged female or a stuffed dummy. The video showed how be unfurled his tail feathers for a split second at the nadir of his dive, which corresponded with a short chirp lasting about 60 milliseconds.
The dive of the males of many species of hummingbird is an important part of its mating ritual. It serves to ward off other males and to attract females, but exactly how they made the sound remained a mystery.
Ornithologists were divided between those who thought the source of the sound was the bird's tiny "song box" in its throat, while others suggested that the short chirp had something to do with its splayed feathers.
The latest study, published in the Proceedings of the Royal Society B, cleared up the controversy by showing that the tail feathers were unequivocally the source of the sound.
When the scientists snipped off the feathers – which grow back in five weeks and do not affect the bird's ability to fly – the males were unable to make the noise. Tests in wind tunnels confirmed that the vanes of the feathers vibrated at the right frequency to make the sound when blown with air travelling at about 50mph.
"Just blowing outward on the tail feather makes the same frequency as in the dive," said Teresa Fao, a member of the research team, who explained that the feather vane vibrated like the reed of a clarinet.
Mr Clark said that the next stage was to identify other species of hummingbird that could have evolved this novel way of "singing" to a mate. Potential candidates include the rufous hummingbird, the tropical woodstar hummingbirds and the bee hummingbird of Cuba.
"Most have funny tail feathers with tapered or narrow tips, all have mating dives and all make a different sound. It's possible that sexual preference by females has caused the shape of the tail feathers, and thus the sound, to diverge, thereby driving the evolution of new species," said Mr Clark.