Vocal tract motor patterns and resonance during constant frequency song: the white-throated sparrow

Bird song is a complex behavior that requires the coordination of several motor systems. Sound is produced in the syrinx and then modified by the upper vocal tract. Movements of the hyoid skeleton have been shown in the northern cardinal (Cardinalis cardinalis) to be extensively involved in forming an oropharyngeal-esophageal cavity (OEC), which contributes a major resonance to the vocal tract transfer function. Here we report that a similar relationship exists between the volume of the OEC and the fundamental frequency in the white-throated sparrow (Zonotrichia albicollis) whose song, unlike that of the cardinal, consists of a series of almost constant frequency notes. Cineradiography of singing sparrows shows that the oropharyngeal cavity and cranial end of the esophagus expand abruptly at the start of each note and maintain a relatively constant volume until the end of the note. Computation of the vocal tract transfer function suggests a major resonance of the OEC follows the fundamental frequency, making sound transmission more efficient. The presence of similar prominent song-related vocal tract motor patterns in two Oscine families suggests that the active control of the vocal tract resonance by varying the volume of the OEC may be widespread in songbirds.