Birdsong: The role of the vocal tract

Abstract

The avian vocal organ, the syrinx, is unique among vertebrates in its morphology and location at or near the base of the trachea. In oscine songbirds, two independent sound sources share the same vocal tract. Despite a growing knowledge of the neural and syringeal mechanisms underlying song, the acoustic role of the suprasyringeal vocal tract in modulating birdsong is still poorly understood. During song, the beak opening, or gape, is typically positively correlated with the dominant frequency of the vocalization, but the mechanism by which beak gape influences vocal tract acoustics is controversial. X-ray cinematography of singing birds reveals song-related movements of the hyoid apparatus and larynx that cause the volume of the upper vocal tract to vary inversely with the songs fundamental frequency. At low fundamental frequencies, the oropharynx and cranial end of the esophagus expand to form a single large chamber, the volume of which is reduced at higher frequencies by collapse of the esophagus and reduction of the oropharyngeal cavity. A computational acoustic model indicates that birds actively tune the volume of their oropharyngeal-esophageal cavity so that its major resonance tracks the fundamental frequency of the song. [Work supported by NIH.]