Role of syringeal vibrations in bird vocalizations

The sound-generating mechanism in the bird syrinx has been the subject of d ebate. Recent endoscopic imaging of the syrinx during phonation provided ev idence for vibrations of membranes and labia, but could not provide quantit ative analysis of the vibrations. We have now recorded vibrations in the in tact syrinx directly with an optic vibration detector together with the emi tted sound during brain stimulation-induced phonation in anaesthetized pige ons, cockatiels, and a hill myna. The phonating syrinx was also filmed thro ugh an endoscope inserted into the trachea. In these species vibrations wer e always present during phonation, and their frequency and amplitude charac teristics were highly similar to those of the emitted sound, including nonl inear acoustic phenomena. This was also true for tonal vocalizations, sugge sting that a vibratory mechanism can account for all vocalizations presente d in the study In some vocalizations we found differences in the shape of t he waveform between vibrations and the emitted sound, probably reflecting v ariations in oscillatory behaviour of syringeal structures. This study ther efore provides the first direct evidence for a vibratory sound-generating m echanism (i.e. lateral tympaniform membranes or labia acting as pneumatic v alves) and does not support pure aerodynamic models. Furthermore, the data emphasize a potentially high degree of acoustic complexity.