How does the poet bring out the immortality of the bird?

While the poet is grieving on the unpleasant aspects of  human life, he cherishes the nightingale's song and feels elated and finds peace in it. He praises the beauty of it. He calls the bird immortal for its humming is not to fade away like humans wither with age and death. He wishes to fly to the nightingale, wants to die and live with the bird. However, Keats soon realises that what he imagines is impossible. Keats is bewitched by nightingale's singing and he sings in praise of this bird whose ageless humming has been heard by the emperors and clowns and even Ruth and by all since times immemorial and will be there to be heard by posterity. Spellbound, he further embellishes and adds the beauty to the bird's song by his personification. Enchanted by the evergreen beauty of the sound of this bird, Keats delays the inevitability of death and lives the immortality of the bird, though not for long.