The imagery of the falcon within the gyre isn’t much removed from the imagery of a pern in the gyre. Could Yeats have misspelled pern (spelling it perne)? If one particular thinks of Yeats’s perne for a chook of prey, then Yeats’ is evaluating the sages to birds of prey. https://elliottpqpon.blogrelation.com/27713454/pernuțe-pentru-copii-can-be-fun-for-anyone