Birdsong Probed With X-Ray Movies

March 27, 2006

X-ray movies of singing cardinals (Cardinalis cardinalis) reveal that songbirds adjust their song’s tonal qualities by actively changing the shape of their upper vocal tract. Humans control their speech through movements in their upper vocal tract, but less is known about how birds control their song. Current hypotheses consider the songbird’s vocal tract as a rigid tube that can alter the song’s tonality only if the bird opens or closes its beak, in much the same way that a flute’s sound can change if a user manipulates fingerholes along the instrument’s length. Tobias Riede and colleagues used x-ray cinematography of male northern cardinals as they sang notes common in cardinal songs. The movies showed that the birds expand their upper vocal tract in a cyclical manner. These movements change the shape of the pharynx and esophagus so that the resonance of those cavities is “tuned” to the song’s lowest harmonic, making this vocal system highly efficient.

“Songbirds, who learn their vocal patterns just as humans do, demonstrate here yet another similarity to humans—that is, in using precise motor control to create complex patterns in the upper vocal tract,” says Riede.

Abstract © 2006 PNAS

“Songbirds tune their vocal tract to the fundamental frequency of their song,” Tobias Riede, Roderick A. Suthers, Neville H. Fletcher, William E. Blevins, PNAS, March 27, 2006

Tobias Riede, Denver Center for the Performing Arts, National Center for Voice and Speech, Denver, CO