What song is he singing? Why?

Maya Angelou’s poem "Caged Bird," first published in her 1983 book Shaker, Why Don’t You Sing?, compares the songs of two birds: the free bird and the caged bird, who sings a song of freedom, pain, trauma, fear, and hope. When taken in the context of the author’s life and other work it can be assumed to be referring to racism and the civil rights movement, but it can also be read as a general discussion of liberation and imprisonment.
The poem refers directly to the song of the caged bird: “the caged bird / sings of freedom” we read in the final verse. The caged bird of the poem has never known freedom but still sings of “things unknown / but longed for still,” despite being fearful and full of rage. This notion of “things unknown” is also important to the nature of his song: he wants to be free, but he doesn’t know exactly what it means to be free. There is a suggestion that the bird can’t help but sing—he senses what he is missing and despite his fear “opens his throat to sing.” Yet his song of freedom is not necessarily a joyful one. It comes from a place of nightmares and death, and holds in it all the trauma of his past. His song of freedom “is / heard on the distant hill,” suggesting that perhaps change will come.
As a metaphor for the life of African Americans compared to white people, the poem emphasizes the different life experience for the two. The caged bird is severely restricted, his opportunities limited. As the second verse shows, “his wings are clipped.” He holds within him the past trauma of all those who have been caged before. Meanwhile the free bird “leaps / on the back of the wind,” is carried downstream by the current, and doesn’t even need to make the effort to fly. He knows it is his right to fly: “he names the sky his own.”
It is worth remembering, too, that Maya Angelou used the image of the caged bird for the title and theme of her 1969 autobiography I Know Why the Caged Bird Sings. The book discusses her early years and how she dealt with racism and traumatic life circumstances.

The caged bird in this poem has been confined to the cage, we must assume, for the whole of his life. He does not know what is beyond the cage because, unlike the free bird, he has never been allowed to "claim the sky" in the same way. However, despite this, he has an innate desire for freedom, and it is about this freedom that he sings.
The bird's song is comprised of "things unknown"—he doesn't know what awaits him beyond the bars of his cage, but this does not stop him from yearning for the "distant hill" and the same freedoms other birds enjoy. He is afraid—his voice contains a "fearful trill"—but his longing exceeds his fear. His song, which represents his desire for freedom, is loud enough to be heard even by those who have already attained that hill in the distance and who are free already.