Sex steroids and their actions on the birdsong system.

It is probably not surprising to most of us that the endocrine system plays a significant role in controlling the singing behavior of birds. We are familiar with the song of birds as a conspicuous acoustic feature of our environment during the avian breeding season. We often witness song when it is produced by birds (males) that are aggressively establishing and defending territories and that are advertising to available females. Thus, it is easy to imagine that song is likely to be stimulated by gonadal hormones. However, the ways in which gonadal sex steroids influence the various parts of the brain at various stages of the bird's life to influence song are complex and far from being completely understood. In this review, I will highlight some of the significant discoveries that have contributed to our view that the songbird brain is a significant and dynamic target of sex steroids. I will also describe what we have learned about properties of the endocrine system and the brain and how they each contribute to making androgens or estrogens available to particular parts of the songbird brain. Finally, I will describe some new research directions that may help answer some unresolved issues about hormonal effects on the songbird brain.