Young Love III - "But, Song, arise thee on a greater wing,"

But, Song, arise thee on a greater wing,
Nor twitter robin-like of love, nor sing
A pretty dalliance with grief - but try
Some metre like a sky,
Wherein to set
Stars that may linger yet
When I, thy master, shall have come to die.
        Twitter and tweet
            Thy carollings
            Of little things,
        Of fair and sweet;
        For it is meet,
            O robin red!
        That little theme
            Hath little song,
            That little head
        Hath little dream,
            And long.
But we have starry business, such a grief
As Autumn's, dead by some forgotten sheaf,
While all the distance echoes of the wain;
Grief as an ocean's for some sudden isle
Of living green that stayed with it a while,
Then to oblivious deluge plunged again!
Grief as of Alps that yearn but never reach,
Grief as of Death for Life, of Night for Day:
Such grief, O Song, how hast thou strength to teach,
How hope to make assay?