It is established that in songbirds the size of several brain song control nuclei varies seasonally, based on changes in cell size, dendritic branching and, in nucleus HVC, the incorporation of newborn neurons. In the developing and adult mammalian brain, the protein doublecortin (DCX) is expressed in postmitotic neurons and, as a part of the microtubule machinery, required for neuronal migration. We recently showed that in adult canaries, DCX-immunoreactive (ir) cells are present throughout the telencephalon, but the link between DCX and the active neurogenesis observed in songbirds remained uncertain. We demonstrate here that DCX labels recently born cells in the canary telencephalon and that, in parallel with changes in HVC volume, the number of DCX-ir cells is increased specifically in the HVC of testosterone-treated males compared with castrates, and in castrated testosterone-treated males paired with a female as compared with males paired with another male. The numbers of elongated DCX-ir cells (presumptive migrating neurons) and round multipolar DCX-ir cells (differentiating neurons) were also affected by the sex of the subjects and their photoperiodic condition (photosensitive vs photostimulated vs photorefractory). Thus, in canaries the endocrine state, as well as the social or photoperiodic condition independently of variation in steroid hormone action, affects the number of cells expressing a protein involved in neuronal migration specifically in brain areas that incorporate new neurons in the telencephalon. The DCX gene may be one of the targets by which testosterone and social stimuli induce seasonal changes in the volume of song nuclei.