Geometry of a hawk's flight

One of the poets featured in the current Poetic Likeness Exhibit -- featuring photographs and paintings and sculptures of poets along with a few favorite lines -- at the National Portrait Gallery is Robert Penn Warren (1905-1989).   Although I hugely admire Warren's novel, All the King's Men, I am not very familiar with Warren as a poet.  The gallery posted, beside Warren's photo, a few lines about a hawk.  And I went searching online to find more.  The exhibit's quote was from Warren's "Mortal Limit" but my search led first to "Evening Hawk" -- with a first stanza bright with geometry;  I offer that stanza here.