How can hormones affect song learning and singing in zebra finches?

In the study of animal learning and zoology, hormones play a critical role in the development and expression of singing behavior in zebra finches (Taeniopygia guttata).1

The influence of hormones on this process is demonstrated through the following mechanisms:

• Neural Development: Early in development, estrogen exposure is essential for females to develop the neural circuitry required for song learning. When females are injected with estrogen early in life, they develop "malelike" brains capable of the learning process.1
• Singing Stimulation: While estrogen facilitates the development of the necessary neural pathways, it is not sufficient to trigger the actual singing behavior. An implant of the male hormone testosterone is required to stimulate these females to sing.1

Consequently, the production of male-typical song in females requires a two-part hormonal process: early exposure to estrogen to organize the brain's neural architecture, followed by later stimulation with testosterone to activate the motor behavior of singing.1