Seasonal changes in the brain nuclei that control song behavior in songbirds are among the most striking examples of plasticity in the adult vertebrate brain. Although seasonal changes in the size of these brain nuclei have been found in several species in captivity, results on seasonal changes in the song nuclei of wild songbirds have been equivocal. In the present study, I measured plasma testosterone (T) concentrations and the size of song nuclei across seasons in wild male rufous-sided towhees (Pipilo erythrophthalmus). I found seasonal changes in both T concentrations and the size of song nuclei that were as large as or larger than those observed in this species in captivity. These results demonstrate that seasonal plasticity of the song nuclei occur in wild, as well as captive, songbirds.