BioEd Online

A bird that lives in the Ecuadorian rain forest attracts mates by striking its wing feathers together behind its back, researchers say.

Birds and other vertebrates usually court partners by expelling air to produce sound. But the male club-winged manakin (Machaeropterus deliciosus) is the first found to use purely mechanical means to produce its 'songs'.

"This is completely unprecedented in the vertebrate world," says Kimberly Bostwick of Cornell University in Ithaca, New York, the lead author of the study, which appears in this week's Science¹. The technique is more common in insects such as crickets.

Only the male manakins have been spotted making noise this way. Their songs sound like two sharp clicks followed by a sustained, violin-like note. That much has been known for a long time: Charles Darwin wrote about the strange manakin sounds² in 1871. He also noted that the male birds had some feather shafts with thickened ends.

Now, more than 130 years later, Bostwick and her colleague Richard Prum of Yale University in Connecticut say that the male manakins use these thickened shafts to sing. Using high-speed digital cameras, they show that the birds strike the tips of their wing feathers together behind their backs in a shivering motion at 106 times a second.

All of a flutter

That's the fastest known limb movement in any vertebrate, Bostwick says. It is swifter than the 90 hertz tail movements of rattlesnakes or the 80 hertz wing movements of hovering cuban bee hummingbirds (Calypte helena).

"It's really fast for a bird that size," says Robert Dudley, a flight physiologist at the University of California, Berkeley, who is not connected to the study.

But it is not fast enough to explain the frequency of the manakin's sound, which at 1,500 hertz is some 14 times faster than the wing oscillations. Bostwick was puzzled when she found the discrepancy. "We had a piece of the puzzle but not the rest."

The answer came when Bostwick took a closer look at the thickened feather shafts. She found that one of the feathers typically has six to eight ridges on it, and its neighbouring feather is bent such that it can rub against it. So during every wing shiver (at 106 hertz), the feather rubs against about seven ridges on the way out, and about seven on the way back.

To prove that this creates the high frequency sound, Bostwick plans to snip off the tip of the neighbouring feather. A bird without that shouldn't be able to make a 1,500-hertz sound.

In the meantime, most experts agree her explanation is correct. "It's hard to explain the outcome in any other way," says Jeffrey Podos, who studies bird acoustics at the University of Massachusetts in Amherst.