What is the caged bird fearful of?

Maya Angelou’s “Caged Bird” is poem about the effects of slavery that echoes a famous poem by Paul Laurence Dunbar called “Sympathy.”
Both poems juxtapose images of two birds, one free and one enslaved. Angelou’s poem includes the following line:

The caged bird sings   with a fearful trill   of things unknown   but longed for still

The word “fearful” could mean several things as it is used here. Angelou might mean that the caged bird fears that his enslavement will forever keep him from enjoying the full human experience. Note that one of the things the bird fears is the “unknown.” While this appears to be a normal human reaction to the unknown, Angelou gives it a twist with the next line, “but longed for still.” Most people fear the unknown and avoid it. The caged bird fears the unknown but desires it, because the unknown is the rest of the human experience that he is denied by slavery.
It is also possible that Angelou is using the word “fearful” to mean “very great,” which is an alternate definition. In this sense Angelou is saying that the caged bird’s song is powerfully expressive of his desire for freedom.