A peek beneath the feathers

Lynn M. Savage, Associate News Editor

The caged bird may know why it sings — but does it know how? In songbirds, sounds are generated by the syrinx — the avian analogue of the human larynx, situated at the lower end of the trachea. The tune-to-be is then modified by the vocal tract and emitted from the beak. Conventional wisdom has it that songbirds have rigid, pipelike vocal tracts and that, therefore, they can change their tune only by opening and closing their beaks and shaping their mouths into various positions.

X-ray cinematography helped image the vocal structural changes that occur within northern cardinals (Cardinalis cardinalis) while singing. Courtesy of Tobias Riede, National Center for Voice and Speech, Denver, and Roderick A. Suthers, Indiana University Bloomington.

However, using x-ray cinematography, investigators at Indiana University Bloomington, at Humboldt University in Berlin, at Australian National University in Canberra and at Purdue University in West Lafayette, Ind., have shown that at least one songbird changes its vocal tract considerably to alter its song as it goes along.

The researchers reported in the April 4 issue of PNAS that they filmed several male northern cardinals (Cardinalis cardinalis) using a mobile x-ray imager made by GE Healthcare of Waukesha, Wis. They acquired 30 frames per second at 1000 x 1000-pixel resolution with the system, which emitted 30 10-ms x-ray pulses per second.

X-ray images of C. cardinalis show the change in volume of the bird’s vocal tract, including the oropharyngeal and esophageal portions, that occurs from the beginning (left; 2 ml) and end (right; 0.6 ml) of a single syllable. Images by Tobias Riede and Roderick A. Suthers. Courtesy of PNAS.

They found that the birds’ singing was accompanied by changes in the shape and size of the upper vocal tract, the so-called oropharyngeal-esophageal cavity, including the end of the esophagus nearest to the head, the pharynx and the oral cavity, supported by movements of the hyoid skeleton — the bony structure that supports this part of the vocal tract. These changes occurred in a cyclical pattern that followed changes in the fundamental frequency produced by each bird. The greater the volume of the air-filled vocal tract, the lower the frequency that was supported.

Positional changes in the larynx and other parts of the vocal tract affect each syllable of a bird’s song. LV = front-to-back path of the larynx; LH = top-to-bottom path of the larynx. Courtesy of PNAS.

A computational model showed that the motor pattern varies the resonance of the oropharyngeal-esophogeal cavity — tuning it to follow the fundamental frequency, increasing its level and making sound production more efficient.

The caged bird now has a little bit more information to go on.