As seen in many poems or poetic texts, there is in "Caged Bird" the use of figurative language, with connotative meaning so that there is a subjective message under the text that must be analyzed under its' context.
The figurative language is a writers' resource for expressing feelings and amplifying the meaning of words.
Let's read an excerpt of Maya Angelou's "Caged Bird" :
"The free bird thinks of another breeze
and the trade winds soft through the sighing trees
and the fat worms waiting on a dawn bright lawn
and he names the sky his own
But a caged bird stands on the grave of dreams
his shadow shouts on a nightmare scream
his wings are clipped and his feet are tied
so he opens his throat to sing.
The caged bird sings
with a fearful trill
of things unknown
but longed for still
and his tune is heard
on the distant hill
for the caged bird
sings of freedom"
With comparing the two birds, we can see how the birds can be a representative symbol of people under different types of life situations, being the caged one a symbol of someone restricted of freedom, wanting their voices to be heard (or needing their rights to be granted, as well).