Using x-rays shows how birds sing
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.