The "Caged Bird" by Maya Angelou describes a bird that first is free and is able to fly everywhere but then he is captured, and his wings are clipped, then he is trapped in cage to prevent him from flying away, however, the bird still sing about freedom, nevertheless, the figurative meaning of this excerpt is that the hope of the bird has being destroyed.
I read this poem not long ago, and I think that excerpt means that the caged bird cant go anywhere, all his freedom is gone except for one thing, and that is singing. He sings because that is the only thing he can do that he is tied down to the cage.
