Keats’ debt to Milton in these lines was well observed. But Keats was not the only poet to borrow from Milton’s description of the swan!
Borrowing from Milton
It was probably Milton who first wrote (at any rate in English) that
“The swan with arched neck
Rows her state with oary feet;”
but Keats has
“The swan, his neck of arched snow,
Oars himself along with majesty.”
“Arching proud his neck, with oary feet
Bears forward;” †
“With snowy pride elate,
Arch their high necks, and row along with state;” ‡
and so on, and so on, till the swan, so dreadfully “oary” does it become, might be a quinquereme rather than a bird.
Phil Robinson (1883). The Poets’ Birds, p. 34. London: Chatto and Windus.
† James Thomson (1766). ‘Spring’ In Works, volume I, p. 31. London: A Millar.
‡ William Broome (c. 1725). ‘Daphnis and Lycidas’. In Samuel Johnson, ed. Works of the English Poets, volume XII, p.17. London: J. Johnson (1810).
I can add some more examples to Robinson’s list:
The swan contented with a humbler fate
Low on the fishy river rows in state.
Elijah Fenton (c. 1707). ‘An Epistle to Thomas Lambard Esq.’. In Thomas Park, ed. (1819). The Select Works of the Minor British Poets, volume II, p. 166. London Suttaby, Evance, and Fox.
His Hair transforms to Down, his Fingers meet
In skinny Films, and shape his oary Feet;
From both his Sides the Wings and Feathers break;
And from his Mouth proceeds a blunted Beak:
All Cycnus now into a Swan was turn’d
Joseph Addison (1717). ‘The Transformation of Cycnus into a Swan’. In Ovid’s Metamorphoses in Fifteen Books, p. 51. London: Jacob Tonson.
in state the snowy swans
Arch their proud necks
William Broome (1739). Poems on Several Occasions, p. viii. London: Henry Lintot.
the feather’d fleet,
Led by two mantling swans, at ev’ry creek
Now touch’d, and now unmoor’d; now on full sail,
With pennons spread and oary feet they ply’d
Their vagrant voyage
William Mason (1778). The English Garden, p. 19. London: J. Dodsley.
Again rejoicing in his length of neck,
With oary feet the swan divides the stream,
And proudly loosens to the coming gale
His snow-white plumage
Henry Headley (1785). Fugitive Pieces, p. 66. London: C. Dilly.
Where silver swans, with snowy pride elate,
Their tall necks mantling, sail’d along in state.
Francis Fawkes (1752). A Description of May, p. 19. London: J. Whiston and B. White.
Borrowing from Douglas?
Fawkes’ poem (quoted above) is a loose translation into English of a poem in Scots by Gavin Douglas, whose lines about the swan do not at first sight resemble Milton’s:
Swannis souchis throw out the respand redis,
Ouer all the lochis and the fludis gray,
Sersand by kynd ane place quhare thay suld lay
Gavin Douglas (c. 1513). ‘A Description of May’. In Fawkes, p. 18.
Nonetheless Fawkes speculated that Milton had read Douglas and used some of the elements in his description of the creation of birds in Paradise Lost, VII.438–446:
That Milton had his eye upon this passage, is plain from his describing the swan, the cock, and peacock, in the order and with several of the attributes, that our author [that is, Douglas] has given them.
Fawkes, p. 20.
Borrowing from Donne?
As pointed out by Peter Shor in comments, Milton’s description of the swan has similarities to John Donne’s.
You must not think me infected with the spirit of Lauder,† if I give you another of Milton’s imitations:
The swan with arched neck
Between her white wings mantling proudly, rows
Her state with oary feet.
“The ancient poets,” says Mr. Richardson,‡ “have not hit upon this beauty; lavish as they have been in their descriptions of the swan. Homer calls the swan long-necked, δολιχοδείρον; but how much more pittoresque, if he had arched this length of neck!”
For this beauty, however, Milton was beholden to Donne; whose name, I believe, at present is better known than his writings:
Like a ship in her full trim,
A swan, so white that you may unto him
Compare all whitenesse, but himselfe to none.
Glided along, and as he glided watch’d.
And with his arched neck this poor fish catch’d—
Progresse of the Soul, st. 24.
Richard Farmer (1767). ‘An Essay on the Learning of Shakespeare’. In Isaac Reed (1809). The Plays of William Shakespeare, p. 111. London: Vernor, Hood and Sharpe.
† William Lauder, who wrote a book claiming that Milton had plagiarised Paradise Lost from passages in a variety of Latin authors, but on inspection, it turned out that Lauder’s quotations were forgeries.
‡ Jonathan Richardson (1734). Explanatory Notes and Remarks on Milton’s Paradise Lost, p. 441. London: James, John and Paul Knapton.
Donne also says of the swan that “It mov’d with state”, which is echoed by Milton’s “rows her state”.
Borrowing from Silius Italicus?
The arching of the neck is unquestionably to be found in Donne, but rowing the oary feet comes from Silius Italicus:—
“Haud secus Eridani stagnis, ripâve Caystri
Innatat albus olor, pronoque immobile corpus
Dat fluvio, et pedibus tacitas eremigat undas.”
William Maginn. In Shelton Mackenzie, ed. (1856). Miscellaneous Writings of the Late Dr. Maginn, p. 259. New York: Redfield.
Maginn is a bit misleading about Silius as a source for Milton, as “eremigat” means “rows” but there is nothing corresponding to Milton’s “oary”:
So a white swan floats on the still waters of the Eridanus or by the bank of Cayster, and lets the current carry its motionless body, while its feet row on beneath the unruffled stream.
Silius Italicus (c. 90). Punica, XIV.189–191. Translated by J. D. Duff (1961). London: William Heinemann.