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 is the climax of the three-line rhythmic buildup. While it technically fits into the anapest / anapest / anapest / iamb pattern introduced by the prior line, it is most comfortably read as anapest / dactyl / dactyl / iamb (although this scheme doesn’t accomodate the word “that) 
 The dactyls in this line are essentially the earlier anapests with their stress moved up.  This change encourages the upward-moving focus of the final lines, and also possibly hints at a kernel of optimism that song (or poetry) can inspire and maintain.
Sympathy