Explain: “his shadow shouts on a nightmare scream”.

The caged bird’s ‘shadow’ gives a sense as if it is powerless or even dead (figuratively). The scream or cry is coming from its shadow instead of the bird itself. This is probably to mean that no one pays any heed to the desperate cry of the bird confined in the cage. Who would care to hear the call of a shadow?

‘Nightmare scream’ is the shout of the caged bird who is having a frightening dream (nightmare) of never getting his freedom from his confinement. He has probably given up on his dreams of liberty. That is why the poet says that the bird ‘stands on the grave of dreams (His dreams are now dead!) / his shadow shouts on a nightmare scream‘.

This is to create a contrast of the caged bird’s life to that of the free bird who ‘names the sky his own’.

This also described that the chances to dreaming sweet dreams are banned by the evil ones. That means the opportunity to sweet-dreaming is died because of the cruel rulings. Right of hoping is also restricted for oppressed people by considering the myths like skin color, caste, class etc.

When one’s reality becomes more adverse than the nightmares, he won’t scream. He is not frightened by the nightmare anymore, but the reality frightens him and does not depict it.
Viewing this from a literary aspect, we can conclude that the shadow which is presumed to be dead is screaming of the harsh reality because the living, caged, perished bird is still longing for the freedom. He wants to live inorder to attain freedom, without depicting any sort of harsh conditions, he is in. This is why his shadow shouts on a nightmare scream.