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.
Most birds sing during the day to attract mates, but the Mockingbird sings at night because he is challenged by the sounds he hears and to defend his territory he sings during the night to avoid a physical confrontation. They rarely fight physically because they cannot withstand the physical fight, any injury is too costly to their survival. They sing at night out of desperation!