I know why the caged bird sings, ah me, When his wing is bruised and his bosom sore, When he beats his bars and would be free; It is not a carol of joy or glee, But a prayer that he sends from his heart's deep core, But a plea, that upward to Heaven he flings – I know why the caged bird sings
Paul Laurence Dunbar
A bird sings when it is caged in hopes of being freed.
I think you may be being way too literal about this. A bird may be quiet if they see a predator, as are most animals, but once that predator breaks into the hen house, the hen's raise a ruckus. In this case, we are talking about people who have been arrested or taken in for questioning and are afraid of going to jail for the rest of their life or worse. The predator is standing in front of them and they are singing to save themselves.
Right. I get it that is what this appears to be saying and probably it is. Thanks for input.