The New York Times,
August 2, 2005Link
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.
This alone would be a remarkable accomplishment for a bird. Hummingbirds typically flap their wings only 50 times a second. But the club-winged manakin's fast shaking alone could not produce the bird's sounds. Its wings produce tones at a frequency of around 1,400 cycles a second -- about 14 times faster than it shakes its wings.
''We had to have some kind of frequency multiplier,'' 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.
This sort of spoon-and-washboard anatomy is unknown in any other vertebrate, but it is well known in insects. Crickets, for example, have ridges on their wings that act like a pick and file when the insects rub their wings together.
''The convergence is simply stunning,'' said Dr. Ronald Hoy, a Cornell expert on insect sounds.
Dr. Bostwick and Dr. Prum reported their findings in the July 29 issue of the journal Science.
The ornithologists plan to test their hypothesis with new experiments. On her next trip to Ecuador, Dr. Bostwick hopes to catch a male club-winged manakin and clip off the raking tip on each wing (a harmless procedure).
''I should be able to completely silence the bird,'' she predicted.
Dr. Bostwick argues that the new research underscores just how powerful sexual selection can be. The mating preferences of female birds can produce not only the peacock's tail or the rooster's crow, but also feathers with microscopic adaptations that let them sing like crickets. ''Darwin would have loved it if he had known,'' Dr. Bostwick said.
Copyright 2005 The New York Times Company