The syrinx of songbirds includes two separate sound sources, the internal tympaniform membranes (ITM), which form the medial wall of each bronchus. The performance of each ITM is controlled by the muscles of that syringeal half. In the canarySerinus canarius, hypoglossal fibers reaching the syrinx via the tracheosyringealis branch of the hypoglossus are responsible for sound modulation. The muscles controlling the performance of the left syringeal half are innervated solely by the left tracheosyringealis; those controlling the right syringeal half are innervated only by the right tracheosyringealis. In the canary and white-crowned sparrow (Zonotrichia leucophrys) a great majority of song elements disappears after section of the left tracheosyringealis, yet remains intact after section of the right one. This phenomenon, earlier described in the chaffinch (Nottebohm, 1970, 1971, 1972) and confirmed in the white-throated sparrow (Lemon, 1973), has been called left hypoglossal dominance. Left hypoglossal dominance occurs in canaries with small or large song repertoires. It occurs in chronically deafened canaries that never had access to their own auditory feedback; it also occurs in birds that had the right or left cochlea removed at an early age. To this extent, left hypoglossal dominance seems to emerge in the individual as a motor phenomenon.