Song sparrow's limited creativity,
Three eighth-notes and a trill all summer long,
The falling second of the chickadee--
It's a pretty humble business, singing song.
Extremely short, I find this poem to be a sort of tongue in cheek commentary about the state of poetry and poets in particular. Given the title, "To The Poets" I imagine that Nemerov is comparing most poets to a song sparrow, who has "limited creativity." The sparrow sings three eighth-notes and trills all summer long. Sure, it may be pretty at first, but it wears thin, and loses novelty. This is a jab at poets of small talent, who are one trick ponies, to mix animal metaphors here.
I think Nemerov wants to take poets down a notch, to remove some of their self-aggrandizement. Poetry, like the singing of birds, should be a more humble business, is what I take away from this. Fundamentally, poets are "singing song." The brevity of this poem underscores its wit, I think. Two and three note birds, poets who are short of ideas but sing all summer long anyways, it all seems to fit together nicely. I got a nice smile out of the poem, and hope that you do as well.