And the faint perfume from its chalice steals —
I know what the caged bird feels!
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!
I know why the caged bird sings, ah me,
When his wing is bruised and his bosom sore,—
When he beats his bars and he would be free;
It is not a carol of joy or glee,
But a prayer that he sends from his heart's deep core,
But a plea, that upward to Heaven he flings —
I know why the caged bird sings!
This line is the turning point of the final stanza, which marks the beginning of a mounting tension created by variations on the mid-stanza rhythm.  This line more or less fits that rhythm, but cues the change with its truncated appearance. 
 In terms of meaning, this line distinguishes the bird’s song from its perch within the cage.  The initial appearance of birdsong in this poem came from the free “first bird,” from the first stanza.  His song isn’t dwelt on, because its motivation is obvious.  In the second stanza, the bird’s perch functions as a tragic approximation of life outside the cage, its use the indulgence of denial.  Dunbar wants to be clear here that the song is different.
Sympathy