The avian vocal system: 3D reconstruction reveals upper vocal tract elongation during head motion.

While birds' impressive singing abilities are made possible by the syrinx, the upper vocal system (i.e., trachea, larynx, and beak) could also play a role in sound filtration. Yet, we still lack a clear understanding of the range of elongation this system can undertake, especially along the trachea. Here, we used biplanar cineradiography and X-ray Reconstruction of Moving Morphology (XROMM) to record 15 species of cadaveric birds from 9 different orders while an operator moved the bird's cadaveric heads in different directions. In all studied species, we found elongation of the trachea to be correlated with neck extension, and significantly greater (ranging from 18% to 48% for the whole motion; and from 1.4% to 15.7% for the singing positions) than previously reported on a live singing bird (3%). This elongation or compression was not always homogeneous along its entire length. Some specimens showed increased lengthening in the rostral part and others in both the rostral and caudal parts of the vocal tract. The diversity of elongation patterns shows that trachea elongation is more complex than previously thought. Since tracheal lengthening affects sound frequencies, our results contribute to our understanding of the mechanisms involved in complex communication signals, one of the amazing traits we share with birds.