Fig. 6. Vocal tract resonances do not vary below 5 kHz regardless of whether the orientation of the maxilla or mandible is held constant (see Fig. 5A for comparison). Spectra obtained under these two conditions vary above 5 kHz, however these differences can likely be attributed to differences in head diameter or differences in vertical head orientation. (A) Spectra obtained for different beak gapes plotted relative to a spectrum obtained with a maximum beak gape (11 mm). (B) Standard deviations (S.D.) calculated across spectra displayed in A.