Dynamic upper vocal tract articulations during zebra finch song – acoustic effects and independence of instantaneous auditory feedback

In birds, the sound generated in the syrinx is modified by upper vocal tract filter properties prior to being emitted. Filtering of upper harmonics, for example, allows birds to produce tonal sounds. The main dynamic filter component is the oropharyngeal-esophageal cavity (OEC), whose volume can be adjusted to track the fundamental frequency of modulated sounds. It is less well understood, how birds, which use harmonically rich sounds, such as the zebra finch (Taeniopygia guttata), engage upper vocal tract structures to produce the complex spectral composition of their vocalizations. Furthermore, it is not known whether birds use instantaneous auditory feedback to adjust filter properties of the upper vocal tract structures. To fill these gaps, we developed a sensor system for tracking expansion of the OEC and recorded these movements together with subsyringeal air sac pressure and vocal behavior in intact zebra finches and after denervation of the right syringeal muscles. To go beyond correlations between OEC expansion and acoustic features, we prevented OEC expansion. The results illustrate the stereotyped dynamics of OEC expansion and confirm that relationships between OEC volume and acoustic features are complex. Significant shifts in sound frequency after denervation did not induce changes in the stereotyped OEC expansion patterns. Preventing OEC expansion caused predicted and unpredicted changes in the spectral composition of song syllables. Together these results illustrate that the complex spectral composition of zebra finch song syllables arises from dynamic adjustments of OEC volume, but resonance features are determined by an interaction of all upper vocal tract structures.