I report on the evolutionary loss of the sternotrachealis muscle (ST) in a vocal bird, the tinamou Darwin's Nothura (Nothura darwinii). Dissections of immature and adult individuals of two subspecies of Darwin's Nothura (N. d. agassizii and N. d. boliviana) and of immature and adult specimens of Ornate Tinamou (Nothoprocta ornata) showed absence of the ST in all the individuals of Darwin's Nothura. By contrast, this muscle was observed in all the specimens of Ornate Tinamou. I provide a description of the morphological organization of the syrinx and the extrinsic muscles of these two species. The presence of the ST in ratites and the Ornate Tinamou suggests that it is primitive to tinamous and a derived evolutionary loss within Darwin's Nothura. The absence of the ST in a vocalizing bird indicates the need to reconsider the function of the extrinsic muscles of the syrinx in phonation. Future research on the morphology of the syrinx of tinamous is required to establish where this loss occurred in tinamou phylogeny.