Four years ago, a Cornell researcher reported a bizarre example of sexual selection in a rare South American bird: The male attracts the female by rubbing specialized wing feathers -- more than 100 cycles per second -- to create a high hum, similar to a sustained violin note.

While the researchers speculated how the sound was created, they have since proven that the club-winged manakin's feathers resonate at a particular frequency to create the tone.

The adaptation is a striking example of a species modifying an essential body part for the purpose of attracting a mate.

"We normally don't think of sexual selection transforming areas of critical importance," said Kim Bostwick, curator of Cornell Museum of Vertebrates and lead author of a study published in the Nov. 11 issue of the Proceedings of the Royal Society.

The sparrow-sized, club-winged manakin has nine inner-wing feathers held adjacent by a ligament on each wing, but uniquely, the two innermost feathers, called the "sixth and seventh secondaries," have enlarged and hollow shafts. The researchers found that when the enlarged sixth and seventh feathers are excited at their resonant frequency -- an object's natural frequency of vibration -- all nine hollow feathers resonate as a unit at 1500 Hertz to create the violinlike note close to an F-sharp. The wing also produces a second harmonic tone at a similar or greater volume as the fundamental tone.

"These feathers have turned into a kind of tuning fork," Bostwick said.

The researchers discovered that all feathers naturally resonate at a frequency of 1500 Hz, but this weak resonance cannot be heard without such special adaptations as the enlarged hollow feathers of the male club-winged manakin, which create the audible tone that attracts females. "The beginnings of that resonance already existed in the feathers," Bostwick said.

Co-authors include Damian Elias, Ph.D. '06, now an assistant professor at the University of California-Berkeley, and Andrew Mason, a former Cornell postdoctoral researcher, now an assistant professor at University of Toronto.

The study was supported by the National Science Foundation and the National Institutes of Health.