I KNOW what the caged bird feels, alas!
        When the sun is bright on the upland slopes;
    When the wind stirs soft through the springing grass,
    And the river flows like a stream of glass;
        When the first bird sings and the first bud opes,
    And the faint perfume from its chalice steals —
    I know what the caged bird feels!

Personal Pronouns - it shows that Dunbar directly shares the emotions of African-American people. He is writing poetry both as a means of expressing his own emotions, as well as being the collective voice of the African-American people.

Extended Metaphor- compares the African-American to a caged bird, who is entrapped and unable to break free due to the bars of the cage that restrict him in doing so. Cages are traditionally symbols of entrapment and denial of opportunities, hence Dunbar's use of cages strongly heightens the sense of imprisonment that was experienced by all African-Americans in his time. Despite the bird's insignificant size, it has the whole sky to embrace, just like an African-American who has endless opportunities to seize, if they were allowed to. However, they are ultimately caged.

Use of words like "sun...wind...river...stream..bud..bird"- creates vivid imagery of the beauty of nature that is clearly out of reach and thus evokes a sense of sympathy in us, knowing the inexpressible struggles that African-Americans experience- they are restricted even the 'free' things in life.

   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! 

Connotations of the words "beat...blood..red..cruel" are strongly negative and relate strongly to slavery and the physical torture which once again evokes sympathy within us, as we in a sense, experience the emotions of defeat and inequality shared by African-Americans.

"cling when he fain would be on a bough-a-swing" - reflects the perpetual suffering and loss of hope experienced by Dunbar's people, for they can only "fain", or pretend to be satisfied with their situation

"old old scars" - repetition of word 'old' reinforces the theme of slavery that is strongly inscribed in African-American history. Dunbar laments the painful past and expresses a defeat in the reality that nothing has changed since then. 

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!

Contrasts between positive and negative connotations ( "carol..joy" and "sore, beats, plea"). This highlights further the sense of being denied of what they should have access to. We as readers are led to consider the pleasant things, and are then encouraged to think of the unfair situation of the African-Americans which further deepens our sympathy for the people and we are encouraged to consider the cruelties of inequality and racial discrimination.

Each stanza ends with a similar sentence but caged bird is doing different things - "feels...beats his wing..sings". Dunbar takes the reader onto a progressive journey. Firstly he decribes the torment and feelings of entrapment and suffering endured, then he protests in futility against the unfair and unequal world. However, in the final stanza, we get the sense that Dunbar remains to hope for a better future, as he 'sings' for freedom.