Explain the imprisonment of the birds in "I Know Why the Caged Bird Sings."

The image of the caged bird in the title is an allusion to a Paul Laurence Dunbar poem entitled "Sympathy."  In this poem, the bird beats its wings against the bars of its cage to no avail and then sings from its heart.  The cage is, of course, a reference to the institutional social and personal oppression that African-Americans suffered during the late Nineteenth and early Twentieth Century.  The bird's song represents the desire to be free from these constrictions.  In the autobiography itself, there are no real references to imprisoned birds apart from this allusion.  However, Angelou's use of this allusion is intended to draw a parallel between the poem's theme and her early childhood experiences as a poor African-American female living in the South during the 1930s and 1940s.  Her "cage" is made of issues arising from class, race, and gender--all of which prevent her from the truest expression of her sense of self.  Her autobiography attempts to chart her growth into an assured young woman who breaks free from these restrictions.