Despair or sadness is the tone that the stanza best supports in "The Caged Bird." The caged bird is singing with a "fearful trill" suggesting a sense of fear or apprehension, while the mention of "things unknown" and "longed for still" conveys a sense of longing and unfulfilled desires. The overall tone of the stanza is one of sadness, as the caged bird is trapped and unable to realize its dreams and desires.