Vocal tract filtering and sound radiation in a songbird.

Bird vocalizations resonate as they propagate through a relatively long trachea and radiate out from the oral cavity. Several studies have described the dynamics with which birds actively vary beak gape while singing and it has been hypothesized that birds vary beak gape as a mechanism for varying vocal tract resonances. Nevertheless, few studies have attempted to quantify the effects of beak gape on vocal tract resonances. We replaced eastern towhee, Pipilo erythrophthalmus L., syringes with a small speaker and obtained recordings of frequency sweeps while rotating each subject in a horizontal plane aligned with either the maxilla or mandible. We describe vocal tract resonances as well as how sound radiates as a function of beak gape. Results are inconsistent with the hypothesis that songbirds vary beak gape as a mechanism for 'tracking' fundamental frequencies in vocalizations. Instead, decreases in beak gape seem to attenuate resonances that occur between approximately 4 and 7.5 kHz. We propose that songbirds vary beak gape as a mechanism for excluding and/or concentrating energy within at least two distinct sound frequency channels.