How does the speaker characterize the bird's singing in "Ode to a Nightingale"?
The nightgale's song is best thought of as a kind of ethereal, other-worldly music of great beauty that can never be fully understood or embraced by man. There is also the sense that man, bound by earthly cares, can appreciate the beauty of the nightingale's music, but can never capture that beauty in poetry. In this sense, the nightingale's song is a reminder of man's mortality.
Here's a quick reading of the poem:
Keats contrasts the beautiful sound of the bird's song with the his own sad mood. The nightingale sings, it seems to the poet, of summertime. This makes the poet think longingly of summer: the famous line, "O for a beakerful of the warm south!" (l. 15) introduces a secondary theme of the poem, which is how the gaiety of the nightingale's song just makes the poet wish to leave his cares behind ("That I might drink, and leave the world unseen, And with thee fade away into the forest dim" l. 20). The nightingale does not know of the troubles of men; the speaker says that he will escape his cares by using poetry to join with the nightingale and his song ("Away! away! for I will fly to thee, / Not charioted by Bacchus and his pards, / But on the viewless wings of Poesy" ll. 31-33), or, in other words, to leave this life behind:
Now more than ever seems it rich to die,
To cease upon the midnight with no pain,
While thou art pouring forth thy soul abroad In such an ecstasy!
Still wouldst thou sing, and I have ears in vain—
To thy high requiem become a sod. (ll. 55-59)
This means that it seems good the speaker to die, because it would ease his pain, but thinks of how his death would be irrelevant to the nightingale, which would sing on after his death. He realizes that the nightingale has a kind of immortal nature -- the song he hears is the same song "heard in ancient days by emperor and clown." (l 64) This thought brings him out of his day dream, and the song of the nightingale fades away, leaving the speaker to wonder, "Fled is that music -- do I wake or sleep?" (l. 80)