Seasonal variation in the size of song nuclei in the brains of male songbirds may be related to the ability to learn to sing new songs as adults. This hypothesis was tested with the rufous-sided towhee (Pipilo erythrophthalmus), a species in which song repertoires are stable after 1 yr of life. Towhees were hand raised in the laboratory and tutored with normal towhee songs. After song repertoires were recorded at 1 yr of age, photoperiods were manipulated so that 10 male towhees experienced short days and 10 males experienced long days. Circulating hormone concentrations and anatomical attributes of song nuclei were then measured. Photoperiod-related differences in the song nuclei of these towhees were as large as those seen in “open-ended learners” (i.e., species that continue to learn new songs as adults). Seasonal changes of the adult song system may thus occur without disrupting existing song repertoires and without the development of new songs. The synaptic plasticity provided by such seasonal variation, however, may enable song learning by adult birds.