The caged bird is singing a song of unknown things which he longs for. Freedom is the subject matter of his song.
He is singing this song to express his feelings and emotions, to stay motivated and also to inspire others.
The caged bird is singing of freedom. He has never experienced it, but longs for it nevertheless. He sings of his hopes and dreams that shall not come true as a result of his captivity.
The caged bird is singing this song to express his feelings. Singing is the caged bird’s only outlet of all his pent up rage.