The syrinx of oscine birds ("true songbirds") is a double vocal organ, and each side has generally been presumed to function independently under separate neural control during phonation. A significant counterexample is demonstrated here in the production of a common vocalization by the black-capped chickadee (Parus atricapillus), in which the 2 acoustic sources of the syrinx interact in a nonlinear fashion. The chickadee produces a sound with multiple frequency components that superficially resemble harmonics. An analysis of vocal production after unilateral and bilateral syringeal denervation shows instead that these frequency components are sum and difference frequencies, or heterodyne frequencies, resulting from cross-modulation between the 2 syringeal sides. A limited form of this bilateral coupling may be achieved after unilateral denervation of either syringeal side but not after bilateral denervation. Unilaterally denervated birds are capable of significant improvement in coupling after 10 d, too short a period for neural regrowth. These results suggest that coupling arises from a passive physical interaction between the 2 syringeal sources which is activated or regulated in some fashion by neural control from either side.