Cineradiography of Singing Northern Cardinals
Cineradiography of singing northern cardinals shows that, by moving the hyoid apparatus and larynx, they actively vary the shape of their suprasyringeal vocal tract during song so that its volume is inversely proportional to the fundamental frequency of the song. In addition to varying the volume of the oropharyngeal cavity, the cranial end of their esophagus is expanded at low fundamental frequencies and can approximately double the volume of the upper vocal tract (Riede, Suthers, Fletcher and Blevins 2006. Proceedings of the National Academy of Sciences; USA . 103:5543-5548). A computational model shows that the bird adjusts the volume of its vocal tract so that the frequency of its first resonance is tuned to the song's fundamental frequency (Fletcher, Riede and Suthers. 2006. Journal of the Acoustical Society of America . 119:1005-1011) The upper vocal tract thus forms a resonance filter that increases the tonality of song by supporting the fundamental frequency while attenuating its higher harmonics. Beak movements may also contribute to the vocal tract filter, but our experiments suggest the acoustic properties of the oropharyngeal and esophageal cavities are more important.
This movie shows the x-ray images of lateral views and ventro-dorsal ("frontal") views of a northern cardinal spontaneously singing the type 1 syllable. First, in the lateral view, the edge of the oropharyngeal-esophageal cavity (OEC) is indicated with a red line. Then, a 3D polygonal model of the cavity is superimposed on the outline which is later filled with a red color. The image of the head and neck is enlarged and rotated. The arrow points to the ceratobranchiale and epibranchiale of the hyoid cornua which moves laterally, expanding the cranial end of the esophagus so that it forms a single cavity with the oropharynx. The lateral asymmetry of the OEC is caused by the fact that the esophagus and trachea lie lateral to each other along part of the neck. The larynx and glottis, through which sound enters the OEC, are located on its ventral surface about half way between its caudal and oral ends (see Fig. 3 A and B ).(From Riede, et al. 2006 PNAS . 103:5543-5548).