Both male and female songbirds hear their species-specific song as nestlings, but only the males of many species sing as adults, and most do so only in spring. Hormones underlie both the difference in song expression between male and female songbirds and the timing of song expression. When investigators injected adult female songbirds with testosterone in spring, those females sang their species-specific song just as males did. Females as well as males form a memory of their species-specific song when they are nestlings, and they have the physical capacity to sing, but under normal circumstances they lack the necessary hormonal stimulation.

How does testosterone cause a songbird to sing? Each spring an increase in circulating testosterone levels causes certain parts of the male’s brain necessary for learning and developing song to grow larger. Individual neurons in those regions of the brain increase in size and grow longer extensions, and the number of neurons in those regions increases. Thus hormones can control behavior by changing brain structure as well as brain function, both developmentally and in response to environmental cues.