Paul Laurence Dunbar uses the visual trope of a caged bird, which is a frequent metaphor for stolen freedom or the sense of being trapped. The trope has been used in literature before and after Dunbar.
It is important to note that his line in the third stanza, "I know why the caged sings," was appropriated by Maya Angelou. It became the title of her memoir in which she recounted being raped as a young girl by her mother's boyfriend. As a result, her uncles kill him. Thus, she chooses not to speak, out of fear that her voice can kill. Angelou applied an ironic inversion to Dunbar's line: she expresses her anguish by refusing to make a sound, while, according to Dunbar's narrator, a bird would sing to express its pain.
Throughout the poem, Dunbar repeats the phrase "I know." This use of anaphora, or the repetition of a phrase for effect, emphasizes his understanding and identification with the small, helpless, trapped creature. This use of parallelism helps Dunbar illustrate the ways in which people, too, can feel small, helpless, and trapped. He expresses this sympathy in the first line: "I know what the caged bird feels, alas!" The exclamation "alas!" indicates that it took some time for the narrator to understand the pain of a caged bird. His newly found understanding is the revelation that is expressed in the poem.
The first stanza illustrates all of the beauty in nature that is withheld from the bird because it is not free. The third and fourth lines are alliterative and seem to evoke the sound of the wind stirring through the grass:
When the wind stirs soft through the springing grass,
And the river flows like a stream of glass;
The second stanza expresses the pain of fighting against captivity. The "cruel bars" on which the "caged bird beats his wing" could be compared to the bars of a prison, one that is both literal and metaphorical. Once it is free, the "pain still throbs in the old, old scars." The memory of captivity lingers, both in the bird's mind and muscle memory.
In the third stanza, the narrator identifies why the caged bird sings: "I know why the caged bird sings, ah me..." The phrase "ah me" is a sigh. Its sound of resignation contrasts with the exclamatory "alas!" The narrator undermines our association of a bird's song with glee, as well as its associations with morning and optimism. Instead, the song is "a prayer...from his heart's deep core...a plea, that upward to Heaven he flings." Here, an allusion is made to Christian faith, which brings solace to blighted ones. This reference, too, is universal in the Western canon.