As we mentioned over in the "Form and Meter" section, this poem is pretty tightly structured in terms of meter and rhyme scheme. But that's not all that's going on here. Close readers will find themselves bouncing back and forth along the sonic rhythms in the poem. Let's take the second stanza as an example:
I know why the caged bird beats his wing
Till its blood is red on the cruel bars;
For he must fly back to his perch and cling
When he fain would be on the bough a-swing;
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! (8-14)
A few things are going on here. First up, there's a lot of alliteration, with all those B words happening in lines 8-9, the F words (not that F word) in lines 10-11, and the P words in lines 12-13. This alliteration creates a rhythm in the poem. It makes us want to read the words out loud, because the alliteration gives a musical edge to the phrases.
At least, that's the glass-half-full way of looking at all this alliteration. A darker take on all these sound echoes is to note that, like the meter and rhyme scheme, all these sound echoes just add to a sense of confinement. Everything is packed in tightly, echoing off itself. Just like the poor bird's cage, this poem's sound traps us (and the speaker) inside.