The vocal tract. The further office which the nose performs, namely, that of contributing to the proper modulation of the voice, depends in great measure on the various cavities and sinuses contiguous to the nasal fossæ.

...the tube of the throat between the larynx and the oral cavity, which is called the pharynx; the oral cavity; and the nasal cavity. These passages are collectively known as the vocal tract.

Beak motions are not necessarily the only means by which birds might vary the acoustic properties of the vocal tract.

Vocal tract

Meaning

Noun