The evolution of the syrinx: An acoustic theory
Fig 4
(A–C) Trachea length was stepwise elongated (to the right of dashed line) or shortened (to the left of dashed line). In chicken and budgerigars, fundamental frequency increases in response to trachea shortening and slightly decreases in response to trachea elongation. In the zebra finch, very small changes were observed in fundamental frequency. However, vocal quality changed sometimes dramatically (e.g., occurrence of nonlinear phenomena). The decrease of fundamental frequency in the inertive reactance region below F1 (primarily in the chicken) is in agreement with theory. Inertance of the vocal tract effectively adds mass to the coupled oscillating system, thereby lowering F0. The closer F0 is to F1, the greater the inertance and hence the greater the F0 drop. Why did the zebra finch apparently exhibit only type 1 coupling throughout shortening and lengthening? There are morphological differences between the structures of the vocal folds of the three bird species that could explain this. The initial length of the trachea is shown by the vertical dashed line. Numerical data used in this figure is included in S1 Data. F1, first formant