It has been suggested that the learning processes which occur during the critical period for bird song development are dependent on adrogens. Castration of male zebra finches (Poephila guttata, Estrildidae) at ages 9-17 days did not prevent song development, and normal learning occurred after the time of castration. Thus the learning processes of song development occur in the absence of gonadal androgens. Castrates tended to develop song more slowly than normals, which is potentially attributable to a decreased rate of singing in the castrates, or to a direct effect of androgen on the learning processes.