"Bluebird" is a poem about a person who hides from himself, afraid to let his sadness show. This inner self is what he calls the "bluebird in my heart." He tries to be strong by keeping his feelings restrained; on the outside, he must be tough so that no one will know the pain he carries on the inside.
At first in the poem, he buries his sadness by drinking and smoking. He builds a facade for the people in his life, keeping his true feelings hidden. He later admits to letting the bluebird out at night and allowing himself to confront his emotions. Even then, he doesn't allow himself to be weak and cry. The secrets he holds inside are too painful for him to fully deal with.
The caged bird imagery is very powerful; it wants to get out. He wishes he could be his true and authentic self, but fear keeps him struggling to control. He is telling himself lies about how tough he is. Only when he is alone in the dark does he let himself feel real. It is nice and a comfort in its own small way, but it is a secret he is trying to keep. The poem is a vehicle for the poet Charles Bukowski to expose himself, as his life was fraught with violence and alcohol abuse.