Paul Laurence Dunbar was the son of former slaves. His father had escaped enslavement by running away. By his mother, the oral tradition of African-Americans was passed on to him. So, when he says “I know what the caged bird feels," he actually knows it.
The agony of the oppressed African-Americans is the most prominent theme of Dunbar's poem “Sympathy.” It’s been expressed through the sufferings of a "caged bird." "The caged bird" has got wings like a free bird, but it can’t use them to fly. Instead, it uses its soft wings to strike against the iron bars hoping to break open the cage. This leaves the poor bird bleeding with sore wings.
…the caged bird beats his wing
Till its blood is red on the cruel bars;
In this way, in a very somber tone, the poet expresses the ordeal of African-Americans. Down the ages, they have been oppressed and enslaved by the whites. The iron bars of racism have prevented them to live a normal life.
Besides, longing for freedom is another important theme running through the poem. The nature in its beautiful forms makes the bird all the more desirous to come out of its cage and fly boundlessly. But, imprisoned and helpless, it can feel the nature only through its sound, sight and smell:
When the sun is bright on the upland slopes;
When the wind stirs soft through the springing grass,
And the river flows like a stream of glass;
When the first bird sings and the first bud opes,
And the faint perfume from its chalice steals—
With the hope that “he would be free,” it beats the iron bars endlessly and prays to the Heaven to end its suffering and liberate it.
Another important theme that cannot be overlooked is that of hope against all odds. Instead of merely waiting for its prayers to be answered or any help to arrive, it continuously strikes against the iron bars with its wings. It stops only when its wings begin to bleed, and, after some time, it restarts again. It never gives up.
And a pain still throbs in the old, old scars
And they pulse again with a keener sting—
I know why he beats his wing!
The blacks, too, have continued their struggle for freedom with this same indomitable spirit through centuries. They won’t stop until they find it.
So, we see that “Sympathy” is about the anguish and pain of the downtrodden African-Americans; it’s about the evil of racism; it’s also about hope and faith, and, about the invincible spirit of the oppressed to persist and carry on.
Paul Dunbar's poem "Sympathy" expresses the understanding that the speaker has with the bird confined to a cage. He sings in the same way that the slaves sang spirituals in the field: music says something to the soul; moreover, it even nourishes the soul.
I know why the caged birds sings,...
When he beats his bars and would be free;
It is not a carol of joy or glee,
But a prayer that he sends from his heart's deep core
The singing, which is natural to a bird, keeps his spirit alive. Just as the bird sings hopefully in his cage, the slaves sang hopefully of their salvation either in this life or in the next. The bird cannot but sing, for if he should stop, he will die; likewise, Dunbar suggests, the African-American must also retain hope that his station in life will improve, for without hope people's hearts and spirit die as there is no reason to continue living. Thus, the theme of "Sympathy" is that hope is the lifeline for the spirit.
This question has already been asked and answered here on eNotes: http://www.enotes.com/paul-laurence-dunbar/q-and-a/what-theme-sympathy-71409
The theme of Dunbar's poem "Sympathy" is about his feelings regarding a life imprisoned. For Dunbar, he recognizes the importance of being free: free to be able to experience the wonders of nature and free to experience the feeling of not giving up.
The bird he describes in the poem is one who must sit in a cage knowing what it is missing. Nature, both around Dunbar and the bird, explodes with freedom and the ability to experience life. Given the bird is caged, it can only look out into life, try to break free, and know that it will only experience life from this singular aspect.
For Dunbar, the comparison between himself and the bird represent the fact that he knows what it is like to live a life caged. He recognizes the pain that it causes trying to break free from the chains of life which bar him from experience. It is through this comparison that Dunbar creates a metaphor for his own life.