The club-winged manakin is the only bird known to sing with its feathers, pictured at top. The bird uses a club-shaped feather as a pick to rake the ridges of another feather. It does this by raising its wings over its back, and shaking them back and forth more than 100 times a second so that one feather rubs the other like a spoon moving across a washboard.

Richard Prum, a Yale ornithologist, was hiking through an Ecuadorean forest 18 years ago when he had one of the strangest experiences an ornithologist can have. He watched a bird sing with its wings.

Dr. Prum was observing a male club-winged manakin. The tiny red-headed bird was hopping acrobatically from branch to branch in order to attract female manakins. And from time to time, the male would wave its wings over its back. Each time the manakin produced a loud, clear tone that sounded as if it came from a violin.

"I was just utterly stunned," Dr. Prum said. "There's literally no bird in the world that does anything that prepares you for it. It's totally unique."

Ever since, Dr. Prum has wondered how the club-winged manakin managed this feat. Now he and a former student, Kimberly Bostwick of Cornell University, believe they have solved the mystery.

Club-winged manakins rake their feathers back and forth over one another, using an acoustic trick that allows crickets to sing. While the technique is common among insects, it has never been documented before in vertebrates.

The noise-making skill of manakins first came to the attention of naturalists in the 1800's. The club-winged manakin belongs to the manakin family (Pipridae), which includes about 40 species, many of which have peculiarly shaped feathers that allowed them to make sounds.

In many species the males use the noises during their courtship displays. "Some of them pop like a firecracker, and there a couple that make whooshing noises in flight," Dr. Prum said.

Charles Darwin was fascinated by manakins. He believed they were a compelling example of how females could cause evolutionary change simply by the influence of their mating preferences - a process he called sexual selection.

If female birds had a preference for males with large tails, for example, males with larger tails would be more successful at reproducing. Darwin argued that the peacock's tail had evolved this way. On the other hand, if females were attracted to noisy males, the males would evolve adaptations that made them noisier - as in the case of manakins.

Biologists have documented the effect of sexual selection in a wide range of animals. Dr. Prum has dedicated much of his career to studying it in manakins. His research shows that wing sounds evolved independently in many manakin lineages. "Mechanical sounds probably evolved a bunch of times in manakins," Dr. Prum said.

The club-winged manakin, with its unique ability to produce musical sounds, was the most extreme example of sexual selection in manakins.

Dr. Bostwick began to study how manakins make their various noises in 1995, when she joined Dr. Prum's lab as a graduate student. In 1997, she traveled to South America to film the birds. On that trip, she saw her first live club-winged manakin.

"I was just blown away by what an odd, odd thing it was," she said.

When Dr. Bostwick returned home, she played her films in slow motion to analyze the manakin wing movements. But the club-wing manakin moved so quickly that its wings were nothing but a blur. "How that motion created that sound was a black box," Dr. Bostwick said.

Over the next few years, this ornithological black box continued to puzzle Dr. Bostwick and Dr. Prum. Dr. Bostwick found a few clues by poring over the preserved club-winged manakins Dr. Prum had brought back from his 1987 trip. She noticed that one feather on each wing had a peculiar feature: its central vane had a series of ridges - seven on average. The club-winged manakin's wing muscles were also remarkably large. "They were like little Popeyes, with big bulging muscles," Dr. Bostwick said.

The clues began to come together in 2002 when Dr. Bostwick returned to Ecuador with a new digital camera that could record 1,000 frames a second, over 30 times faster than her previous model. She made new films of the club-winged manakin, and when she returned home she found that she could finally see what the bird's wings were doing. It turns out that when the bird raises its wings over its back, it shakes them back and forth over 100 times a second.