The portrait below was made shortly before he died.
There's also my own question, "Is this so--or is it just poetry?
(5) The question of historical correspondences is, however, irrelevant to Keats.
In his ode, the important thing is that the Phoenix and Turtle represent Beauty and Truth as spiritual or Platonic absolutes, a signification actually aided, when he wrote, by the allegorical birds eluding historical identification.
She may well wish him to catch her--but the important thing is that on the urn, the music is silent and the chase, perpetual.
And of course, it's worth remembering here that the function of an urn is to hold for eternity the ashes of the dead.
In writing his "Ode on a Grecian Urn," Keats seems to be meditating on "The Phoenix and Turtle" almost to the extent that he is, within the fiction of the ode, contemplating an urn.
While describing and imaginatively engaging scenes depicted on the urn, he is giving expression or, rather, re-expression to Shakespeare's poem.
That Shakespeare is Keats's source ought to be clear to anyone familiar with the ode who reads "The Phoenix and Turtle"--a relatively short poem, only seventeen lines longer than Keats's ode.
Their relationship has not been noted in print prior to my earlier, abbreviated version of this essay in the TLS, probably owing to Shakespeare's poem being little taught because curricular emphasis is on the plays and sonnets and maybe because its allegorical correspondences are unknown (Dilworth 15).