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. 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. Birds regulate the airflow through the syrinx with muscles—M. syringealis dorsalis and M. tracheobronchialis dorsalis—that control the medial and lateral labia in the syrinx, whose action may close off airflow. Song may, hence, be produced unilaterally through one side of the syrinx when the labia are closed in the opposite side. Early experiments discover lateralization Lateral dominance of the hypoglossal nerve conveying messages from the brain to the syrinx was first observed in the 1970s. This lateral dominance was determined in a breed of canary, the waterschlager canary, bred for its long and complex song, by lesioning the ipsilateral tracheosyringeal branch of the hypoglossal nerve, disabling either the left or right syrinx. The numbers of song elements in the birds’ repertoires were greatly attenuated when the left side was cut, but only modestly attenuated when the right side was disabled, indicating left syringeal dominance of song production in these canaries. Similar lateralized effects have been observed in other species such as the white-crowned sparrow (Zonotrichia leucophrys), the Java sparrow (Lonchura oryzivora) and the zebra finch (Taeniopygia guttata), which is right-side dominant. However, denervation in these birds does not entirely silence the affected syllables but creates qualitative changes in phonology and frequency. Respiratory control and neurophysiology In waterslager canaries, which produce most syllables using the left syrinx, as soon as a unilaterally produced syllable finishes, the right side opens briefly to allow inspiratory airflow through both bronchi before being closed again for left syrinx song production. During this “mini-breath” the left side may remain partially or fully adducted, allowing less inspiratory airflow than the right side while remaining ready to quickly resume singing. 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. This indicates that while inspiration and syringeal