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.
Dr. Bostwick traveled to New York to study the manakin collection at the American Museum of Natural History. "I spent a lot of time playing with the feathers," she said. She noticed that next to the strangely ridged feather was another feather with a stiff, curved tip. She realized that each time a manakin shook its wings, its tip rakes across the ridges of the neighboring feather like a spoon moving across a washboard. Each time it hit a ridge, the tip produced a sound. The tip would strike each ridge twice - once as the feathers collided and once as they moved apart again.
Dr. Bostwick realized that this raking movement allowed a wing to produce 14 sounds during each shake. As a result, a bird could shaking its wings 100 times a second could produce a sound with a frequency of 1,400 cycles a second. "All the questions that hadn't made any sense just clicked into place," Dr. Bostwick said. (More from the NY TIMES)
* Cornell Press Release