Hypotheses are reviewed about how the vocal apparatus of birds operates during singing, focusing especially on the means by which the tonal sounds so typical of birdsong are generated. Evidence for the widely held view that the "two voices" in the songbird syrinx act independently of one another, and independently of acoustic resonances of the vocal tract, is found to be incomplete. Results of several experiments, including effects of placing singing birds in helium air, indicate that coupling of the "two voices" can occur and that changes in vocal tract resonances modify the tonal quality of birdsongs. A new model of songbird phonation is proposed, implying close coordination between the operation of the acoustic sources in the syrinx and dynamic modifications of the vocal tract.

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