Beautiful and cunningly constructed.
She begins with the dumb things that fly – and she has a small pun here with “Hours” that fly past. And I call them dumb in the sense that there is no “Elegy” for them when they die, unlike the final stanza which implies that she is wondering if there will be an Elegy (a place in “the skies” for her soul); this is the “Riddle” that she lies under looking up at. Once again she is equating nature imagery with things of this world as opposed to what lies beyond – which might seem like an obvious thing to say, but she has a strong attachment to this world and the beauty in it, and you can feel her appreciation of this world as the “Hours” fly past as she is lost in contemplating / appreciating them.
This leads into what they fly over (the earthly things on the ground). What’s interesting is her movement from “Hours” moving to “Eternity” which does not move but rather “stay[s]”. The unusual word choice of “behoove” made me think that she is recognizing that these images represent, for her, what the physical universe will require from all of us, but this also implies the separation of body (nature) from soul (spiritual) which happens at death, but in life the two are commingled, which she seems to question as being part of the “Riddle” she is trying to “expound”. How can a person be both physical AND spiritual? How can time be both made up of hours AND be eternal?
Up to this point I was curious about the word “Elegy”. She ties this into “Grief”, but and elegy is musical (usually at a funeral – and she alludes to the mound of a fresh grave with her use of “Hills” between “Grief” and “Eternity”). This musical component makes the line “These are that resting, rise” take on a musical note where upon death the body rests just as in music a rest is a pause, but the next word is “rise” which implies the music will begin anew, as well as alluding to Christ rising from the dead. This rising / flying also recalls “Once more, my now bewildered Dove” when Noah tossed the Dove to find land – we can imagine her soul rising heavenward as a bird would fly, only her bird is not a simple animal but is her spiritual being taking flight like a bird searching for home (dry land).
Finally she tries to reconcile the two states of the first two stanzas but it remains a “Riddle” to her because she hasn’t figured out how one makes the leap from “rest” to “rise”, though the key might be in the punctuation because she uses a comma between “rest” and “rise” which basically means ‘and’ thus they are two states of the chain of being – they are not different things, but are the same and thus connected.