Birdsong, usually males only (1 Viewer)

I know that in many, if not most cases, it is the male bird of any given species that sings. Which brings my question, is this due to hormonal differences, or are female birds of those species actually incapable of singing? On a purely hypothetical standpoint, if a female Varied Thrush(a local species that is a favorite of mine)were injected with whatever seperate hormone the male posseses, would she start to sing?

I hope this isn't too odd of a question.