FUNCTIONAL lateralization of the brain, once considered unique to human language, remains poorly understood(1-5). The most convincing example of this phenomenon in animals is singing behaviour in songbirds(1,4), in which asymmetries range from a clear unilateral dominance(6-8) to approximately equal contributions from each side of the vocal organ, the syrinx(9). Here we report that in brown thrashers (Toxostoma rufum) only the activity of muscles that gate sound production by regulating airflow through each side of the syrinx is lateralized. Other syringeal muscles that primarily control the phonetic structure of vocalizations are active on both sides of the syrinx. This explains the puzzling absence of laterality in the morphology(10,11) and activity(12,13) of the higher central song control nuclei and suggests that song lateralization did not evolve as a means of achieving a single ’executive’ command centre, or as a way of economizing on motor circuits to free brain space for other tasks(14,16).