In "Caged Bird," what does the line "and his shadow shouts on a nightmare scream" mean?

Maya Angelou’s 1983 poem “Caged Bird” compares the plight of a caged bird to the flight of a free bird. Many readers have interpreted Angelou’s poem as an extended metaphor with the caged bird representing the historical struggles of African Americans.

The line above is in the 5th stanza, which describes the caged bird who “stands on the grave of dreams/ his shadow shouts on a nightmare scream/ his wings are clipped and his feet are tied/ so he opens his throat to sing.” The grave of dreams can refer to a person who has given up on his dreams. The shadow, rather than the bird itself, shouts, revealing a sense of powerlessness, for who would hear the shout of a shadow? This contrasts with the free bird described in the previous stanza who boldly “names the sky his own.”

The caged bird’s “nightmare scream” gives an otherworldly sense that, again, the cry will not be heard. The words “shadow” and “nightmare” evoke a dark outlook, where only the bird’s shadow or nightmares may escape the confines of the cage.