From Feathers, a Violin
High-speed video and the physics of sound show how the Club-winged Manakin sings without its voice
Most frogs, birds, and mammals communicate by vocalizing, moving air over specialized structures in their respiratory systems to croak, sing, howl, or--in the case of humans--speak in words. Some vertebrates use other sounds too--whistling, clapping, drumming, or rattling, for example. The rattlesnake sends a threatening message by rattling its tail, a Ruffed Grouse produces a dull thudding sound with its wings to court a mate, and a woodpecker drums out its territorial signal on a hollow snag. Examples of special whistling sounds made by feathers include the winnowing display of a snipe and the tinkling notes of a displaying woodcock.
Birds often use their wings and other body parts to make sounds, but the planet's preeminent wing-popping, -clicking, -snapping, and -rattling birds are small, colorful manakins that live in tropical forests from Mexico to Argentina. There are about 40 species of Neotropical manakins (Pipridae), 20 of which make nonvocal sounds, or sonations. Male manakins take the prize for the most diverse and interesting nonvocal sounds produced in the bird world.
However, even among manakins, one species stands out: the Club-winged Manakin (Machaeropterus deliciosus), the only bird that uses its feathers as a violin. Recently, I published a paper with my former graduate advisor, Richard Prum of Yale University, in which we proposed an explanation of how this species makes a uniquely tonal sound with a small set of specially shaped feathers (Science, July 29, 2005).
The small, chestnut-colored males with their cherry-red caps and black-and-white wings look fairly ordinary, but they make an extraordinary sound. Male Club-winged Manakins make a ringing tick, tick, ting sound when trying to attract females to mate. The ting in particular is a high-pitched note, sounding somewhat like a violin. To make the sound, the bird tips forward and flips its wings above its back. The wings look a little blurry as they are held above the back, but it wasn't clear exactly how this odd sound is made.
I recorded males making this sound in the Ecuadorian Andes using a digital high-speed video camera. By examining the video at slower speeds, I could see that the males were knocking a pair of modified wing feathers together over their back at a very high rate--more than 100 cycles per second--twice as fast as an average hummingbird flaps its wings. Although the video showed feathers knocking together, the sound the bird made was a tone. Further, the sound produced by these knocking feathers was approximately 14 times faster than the rate of the knocking itself. There had to be more to the story than what was visible in the video.
As a curator of the Cornell University Museum of Vertebrates, I often use museum specimens in my research. After months of examining Club-winged Manakin feathers from specimens and reading about how insects produce sound, I noticed seven tiny ridges on the club-shaped, hollow wing feathers for which the Club-winged Manakin is named. An adjacent feather had a kink in it that makes its feather shaft lie on top of the ridges. I had discovered the violin and bow on the feathers of the Club-winged Manakin.
The violin is a refined instrument. Producing sound with a violin is more complicated than drumming (beating two things together) or whistling (simply forcing air through thin structures). Violins involve friction. Violins have resonance chambers. Violins require one structure moving across another structure, like a bow across a string. Violins involve “frequency multiplication,” a fancy term used to convey that one motion, such as the drawing of a bow across a string, translates to many motions, such as the fine sticking and slipping of the string on the bow which creates thousands of vibrations in the string itself.
The Club-winged Manakin is a bird with a pair of violins built into its wings. The wide, ridged feathers form the body of the violin and the thin, kinked feathers next to them forms the bow. When male manakins knock their wings together across their backs over and over again in a rapid cycle, the momentum from the heavier-than-usual feather shafts causes the bow feather to slide first inward across the ridged feather, then outward. This generates friction and vibration. The seven ridges generate seven knocks on the way in and seven on the way out, which adds up to that frequency multiplier of 14 needed to explain the discrepancy between the rate of feather knocking and the frequency of the sound heard. The fat hollow feather resonates at this stimulated frequency, and out comes the unique sound of the Club-winged Manakin.
Although a few insects have made violins of their bodies to communicate, there are no other violins among vertebrate animals besides the stringed instruments invented by humans. Thus, male Club-winged Manakins use a unique mechanism to communicate their attractiveness to potential mates.
Kimberly Bostwick is curator of the bird and mammal collection in the Cornell University Museum of Vertebrates.