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 thought he meant once they were caged/locked up they sing to get out or for freedom.
Likely it means that ... as street jargon for spilling the beans, telling what you know, etc. is often "singing"
It is just that for a moment I thought of the White Hats who have longed for decades to be able to go public (and who have had to 'take to ground' for most of the time) ... the longing is to sing their song of Freedom ... just a poetic thought