Precise coordination across hemispheres is a critical feature of many complex motor circuits. In the avian song system, nucleus RA plays a key role in such coordination. It is simultaneously the major output structure for the descending vocal motor pathway and it also sends inputs to structures in the brainstem and thalamus that project bilaterally back to the forebrain. Because all birds lack a corpus callosum and the anterior commissure does not interconnect any of the song control nuclei directly, these bottom-up connections form the only pathway that can coordinate activity across hemispheres. In this study, we show that unilateral lesions of RA in adult male zebra finches permanently disrupt the birds stereotyped song. In contrast, lesions of RA in juvenile birds do not prevent the acquisition of normal song as adults. These results highlight the importance of hemispheric inter-dependence once the circuit is established but show that one hemisphere is sufficient for vocal behavior when interdependence is prevented during a critical period of development. The ability of birds to sing with one RA provides the opportunity to test the effect of targeted microlesions in RA without confound of functional compensation from the contralateral RA. We show that microlesions cause significant changes in song temporal structure and implicate RA as playing a major part in the generation of song motor patterns. These findings implicate a dual role for RA, first as part of the program generator for song, and second as part of the circuit that mediates interhemispheric coordination.