The right and left hypoglossal nerves provide the motor innervation to the syrinx. The mechanism of song production in the chaffinch, Fringilla coelebs, is inferred from losses of song components following unilateral denervation of the syrinx. Greenwalt ('68) and Stein's ('68) “two-voice” theory of song production is supported. An independent sound source in each bronchus is under the control of the hypoglossal innervation to that side of the syrinx. Since a bird can silence a denervated bronchus, this is probably done by reducing air flow through its bore.

Bilateral denervation of the syrinx renders a chaffinch virtually aphonic. The effect of unilateral denervation depends on the stage of vocal development when this operation is conducted, and on whether the right or left hypoglossus is cut. In adult birds section of the left hypoglossus results in the loss of most of the song's components. Section of the right hypoglossus results in the loss of few, if any, components. These losses are irreversible. The same operation in birds in the plastic song stage results in partly reversible changes, so that some of the lost song components can be redeveloped by the intact nerve. When either the right or the left hypoglossus is cut before the bird has started developing its song, normal song will develop under the control of the intact side. These observations are interpreted in terms of neural lateralization of vocal control and loss of motor plasticity. Loss of motor plasticity probably determines the end of the critical period for song learning.

It is known that in the chaffinch song develops to match an acquired auditory model or template. The availability of this template is evaluated in the light of song recovery after unilateral hypoglossectomy during the plastic song stage. It is suggested that as song development proceeds, the template is recalled with greater and greater precision. Motor disruption of vocal development may render this template inaccessible.