The twentieth-century African-American poet Paul Dunbar wrote a poem, "Sympathy," about a caged bird, in which he observes that, although it is caged, by its very nature the little creature must sing; but above that, the bird sings to keep himself alive. Similarly, then, in John Keats's eloquent "Ode to a Nightingale," the sweet bird sings because he belongs to innocent Nature that is not among the weary and feverish; "Where but to think is to be full of sorrow."
Much as in the gospel of St. Matthew, the nightingale is not anxious as is Keats who feels the "leaden-eyed despair":
Therefore I tell you, do not worry about your life, what you will eat or drink; or about your body, what you will wear.... 26 Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they? 27 Can any one of you by worrying add a single hour to your life? (6:25-27)
As a Romantic poet, John Keats placed much interest in the imagination, which connects intuitively with Nature, transcending the mundane and elevating the soul to new heights. It is, therefore, intrinsic to the nightingale to sing, to bring joy to his listener who imagines the branch upon which he perches, and the flowers at his feet. It is the nature of the bird to feel none of the anxiety of Keats whose beloved have died; it is the nature of the bird, with his lovely melody, to lighten the heart and elevate the soul of the poet.