We describe song geographic variation (n = 61) of the Brown-capped Redstart (Myioborus brunniceps), a montane parulid of the eastern slope of the Andes of Bolivia and Argentina, and explore song divergence association with geographic isolation and ecological factors. We scored temporal and spectral song metrics through standard methods, reduced variation through principal component analysis, and used generalized linear models to evaluate the association of song with geographic isolation, vegetation, and climate. We found that the songs of this bird can be divided into 2 dialects: a fast note rate song (at Andean forests) and a slow note rate song (at the San Luis and Córdoba hills), with a maximum pitch and note rate in northern Argentina. Note rate variation followed a geographic cline from northern Bolivia to northern Argentina, which changed at the southernmost region of the study area. We found that variation of song spectral features is associated with environmental factors, suggesting the action of selective forces on character evolution, while song temporal aspects were strongly associated with geographic isolation and weakly associated with environmental aspects, suggesting the action of both drift and selective forces. A question that remains to be answered is the role of body size on song variation of this species. The song of the Brown-capped Redstart followed a pattern of geographic variation that is often observed in oceanic islands, suggesting that such patterns (insularity syndrome) would also evolve on continental islands of habitat.