I assume that once again, what I'm getting out of the poem has nothing to do with your intentions, John, and that I'm about to make a fool of myself. But here goes:
I am struck by the comparisons/contrasts (beginning with the poem's telling us what the thing that landed on the roof was not), and also by the many references to time.
A hawk-sized flying animal with teeth did, indeed, exist, although not at the same time as humans. See
https://en.wikipedia.org/wiki/Pterod...yniuk_wiki.png
It seems to me that the man in the poem could regard two great expanses of time—not only the one before him, but also the one behind him. The concept of eternity makes the dividing line between the two less significant, doesn't it? And if the conditions are right, evolution might perhaps return to a place it has been before, however unlikely that might be.
I hope some of these thoughts are helpful.