November 11, 2009
Solving a longstanding puzzle among bird experts, scientists have found that the sharp, violin-like sounds of a South American songbird come not from the beak but from a suite of specially evolved, vibrating feathers.
A new study offers the first hard evidence that birds use feathers for audible communication as well as for flight and warmth.
In 2005 Kimberly Bostwick theorized that the male club-winged manakin—a tiny bird of the Andean cloud forest—was vibrating a club-shaped wing feather against a neighboring, ridged feather to "sing" when trying to attract females. (See "Cloud Forests Fading in the Mist, Their Treasures Little Known.")
Proving the feather-song connection, though, would be a huge challenge.
"It was very hard to mess with the birds' feathers and still have them do their display," said Bostwick, curator of birds and mammals at the Cornell University of Vertebrates in Ithaca, New York.
"There were many times where I listened to the sound and started doubting that a feather could possibly make [the sound]," she recalled.
To determine, once and for all, how the manakin was making its bizarre sounds, Bostwick and colleagues decided to take feather samples and analyze them in a lab.
She knew from previous work that the frequency of the sound made by the manakin was 1500 hertz—1,500 cycles per second. If the two feather types were making the sound, they should resonate when vibrated at the same frequency during the experiments.
The team used lasers to monitor vibrations as they were oscillated by a lab device called a mini-shaker. The special feathers vibrated at exactly 1500 hertz—proving they're responsible for the strange sounds.
But there's a twist: Bostwick was surprised to find that club and the ridged feathers aren't a duet, but part of a chamber orchestra.
Individually the manakin's "regular" feathers didn't resonate like the special ones. But when the nine feathers closest to the special feathers were still attached to the ligaments, they vibrated at around 1500 hertz, harmonized with the club feathers, and amplified the volume of the sound.
The results, Bostwick said, could lead to better understanding of the newly discovered form of bird communication.
Lots of birds make simple clapping sounds or whooshing noises with their wings, and we haven't even begun to understand how the sounds are made or how they've evolved, she added.