Passerine birds produce song through the vocal organ, the syrinx, which is composed of bilaterally symmetric halves located where the trachea separates into the two bronchi.
Using endoscopic techniques, it has been observed that song is produced by air passing between a set of medial and lateral labia on each side of the syrinx.
[1] Song is produced bilaterally, in both halves, through each separate set of labia unless air is prevented from flowing through one side of the syrinx.
syringealis dorsalis and M. tracheobronchialis dorsalis—that control the medial and lateral labia in the syrinx, whose action may close off airflow.
[6] However, denervation in these birds does not entirely silence the affected syllables but creates qualitative changes in phonology and frequency.
[2] When bilateral airflow and subsyringeal air sac pressure were monitored along with electromyographic activity of expiratory abdominal muscles in brown thrashers (Toxostoma rufum), it was observed that during unilateral production of song, expiratory abdominal muscle activity was the same on both sides.
[2] Waterschlager canaries with left tracheosyringeal nerve cuts are only able to produce up to 26% of the pre-operation syllable repertoire.
[12] Possibly explaining their strong left lateralization, canaries of the waterschlager strain contain an inherited auditory defect that decreases their sensitivity by up to 40 dB to sounds higher than 2 kHz, which are produced mainly by the right side of the syrinx.
[16][18] This indicates that precise unilateral control of song production in the syrinx of certain birds, such as cardinals, has allowed them to become unique vocal specialists.