In this poem, when comparing the caged bird to suppressed black people in America, is the poet using metaphor or symbol?
Metaphor and symbolism are related, but in symbolism, the thing another object stands for is generally more complex and abstract than in metaphor. Therefore, in this case, the bird itself represents oppressed black people, so metaphor is used for the specific case you ask about. Here is some information that supports this contention:
The Difference Between Metaphors and Symbols - The Sceptical Prophet