A NEW MECHANISM OF SOUND GENERATION IN SONGBIRDS

Our current understanding of the sound-generating mechanism in the son gbird vocal organ, the syrinx, is based on indirect evidence and theor etical treatments. The classical avian model of sound production postu lates that the medial tympaniform membranes (MTM) are the principal so und generators, We tested the role of the MTM in sound generation and studied the songbird syrinx more directly by filming it endoscopically . After we surgically incapacitated the MTM as a vibratory source, zeb ra finches and cardinals were not only able to vocalize, but sang near ly normal song. This result shows clearly that the MTM are not the pri ncipal sound source. The endoscopic images of the intact songbird syri nx during spontaneous and brain stimulation-induced vocalizations illu strate the dynamics of syringeal reconfiguration before phonation and suggest a different model for sound production. Phonation is initiated by rostrad movement and stretching of the syrinx. At the same time, t he syrinx is closed through movement of two soft tissue masses, the me dial and lateral labia, into the bronchial lumen. Sound production alw ays is accompanied by vibratory motions of both labia, indicating that these vibrations may be the sound source. However, because of the low temporal resolution of the imaging system, the frequency and phase of labial vibrations could not be assessed in relation to that of the ge nerated sound. NevertheIess, in contrast to the previous model, these observations show that both labia contribute to aperture control and s trongly suggest that they play an important role as principal sound ge nerators.