Cortico-basal ganglia (BG) circuits are thought to promote the acquisition of motor skills through reinforcement learning. In songbirds, a specialized portion of the BG is responsible for song learning and plasticity. This circuit generates song variability that underlies vocal experimentation in young birds and modulates song variability depending on the social context in adult birds. When male birds sing in the presence of a female, a social context associated with decreased BG-induced song variability, the extracellular dopamine (DA) level is increased in the avian BG nucleus Area X. These results suggest that DA could trigger song variability changes through its action in Area X. Consistent with this hypothesis, we report that DA delivered to Area X weakens the output signal of the avian cortico-BG circuit. Acting through D(1) receptors, DA reduced responses in Area X to song playback and to electrical stimulation of its afferent cortical nucleus HVC (used as a proper name). Specifically, DA reduced the response to direct excitatory input and decreased firing variability in Area X pallidal neurons, which provide the output to the thalamus. As a consequence, DA delivery in Area X also decreased responses to song playback in the cortical output nucleus of the BG loop, the lateral magnocellular nucleus of the anterior nidopallium. Further, interfering with D(1) receptor transmission in Area X abolished social context-related changes in song variability. In conclusion, we propose that DA acts on D(1) receptors in Area X to modulate the BG output signal and trigger changes in song variability.