Secret of Darwin's 'violin-playing bird' revealed
The club-winged manakin, which was described by Darwin in his book on sexual selection called The Descent of Man, uses its wings to produce a baleful humming sound to attract a mate. It is the first time that ornithologists have filmed the "wing song" of the small South American bird as it beats its wings 100 times a second - twice as fast as the wing-beat of a hummingbird.
High-speed video recordings taken with a digital camera and played back in slow motion have revealed how males of the club-winged manakin, Machaeropterus deliciosus, lean forward to raise their wings behind their backs before flipping them to make the humming tune.
The sound it produces, which is described as "tick-tick-ting", is as loud as ordinary birdsong and can be heard tens of yards away. The "tick" notes are sharp, tonal clicks, whereas the "ting" is a sustained, violin-like note which lasts a third of a second.
Kimberly Bostwick of Cornell University in Ithaca, New York, and Richard Prum of Yale University in New Haven, Connecticut, describe how the manakin makes its courtship sound in the journal Science. They believe that the rapid movements of the wings, combined with the unusual structure of some of the bird's wing feathers, sets up a resonating note within the hollow shafts of the feathers.
Dr Bostwick said that the male had evolved the elaborate sound-producing mechanism because of the intense sexual competition between males of the species to attract females - the same sort to sexual selection that resulted in the peacock's tail.
"Essentially an instrument has evolved in this species, in this case a refined instrument. In general, if an adaptation is really weird and out there, it is produced by sexual selection," she said.
When the scientists studied the wings of the birds they found that each wing had one feather with a pick, or small blade, and an adjacent feather with ridges and enlarged, hollow shafts.
Each wing created its own violin-like hum as the pick feather rubbed against the adjacent ridged feather during the high-speed flapping of the wings.
"We rule out a lot of hypotheses, but when I realised that the wing feathers were twisted in a way that forced them to rub, I knew we had something," Dr Bostwick said.
Club-winged manakins breed in "leks", an arrangement where competing males display their courtship signals to females who then decide which partner to choose. This produces intense competition between males to attract a mate, with one male often mating with most females.
There are 40 kinds of manakins but not all of them use sound as the primary mating signal. One species has developed a sophisticated dance which involves gliding up to a female without appearing to move his feet.
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