Role of syringeal vibrations in bird vocalizations

The sound–generating mechanism in the bird syrinx has been the subject of debate. Recent endoscopic imaging of the syrinx during phonation provided evidence for vibrations of membranes and labia, but could not provide quantitative analysis of the vibrations. We have now recorded vibrations in the intact syrinx directly with an optic vibration detector together with the emitted sound during brain stimulation–induced phonation in anaesthetized pigeons, cockatiels, and a hill myna. The phonating syrinx was also filmed through an endoscope inserted into the trachea. In these species vibrations were always present during phonation, and their frequency and amplitude characteristics were highly similar to those of the emitted sound, including nonlinear acoustic phenomena. This was also true for tonal vocalizations, suggesting that a vibratory mechanism can account for all vocalizations presented in the study. In some vocalizations we found differences in the shape of the waveform between vibrations and the emitted sound, probably reflecting variations in oscillatory behaviour of syringeal structures. This study therefore provides the first direct evidence for a vibratory sound–generating mechanism (i.e. lateral tympaniform membranes or labia acting as pneumatic valves) and does not support pure aerodynamic models. Furthermore, the data emphasize a potentially high degree of acoustic complexity.