The "caged bird" stands for none other than the oppressed blacks. Devoid of liberty and basic human rights, the blacks have led hellish lives, full of pains and sufferings, for centuries. Its song of freedom demonstrates the rage and optimism of the blacks that toughen them to endure.
Although the caged bird “sings of freedom,” she sings “with a fearful trill.”
The dream of liberty has been seen by the blacks for ages. The poet’s uncountable ancestors have spent their whole lives hoping to see the light of freedom. This discomforting sense of undergoing persecution for years is well evoked in the following lines:
But a caged bird stands on the grave of dreams
his shadow shouts on a nightmare scream
The blacks' dream of liberty is very old. Despite their continued struggle, they have suffered defeat and frustrations repeatedly.
Thus, the caged bird’s fear is about the uncertainty of achieving freedom in the future. Its fears reflect those of the blacks who no more wish to go through the pains of racism, discrimination and bestial treatment at the hands of the whites. The blacks are scared of the darkness hanging over the lives of their offspring.
The word “nightmare” is suggestive of the blacks’ unspeakable suffering and “scream” reflects their expression of agony.