The poem begins like a field guide, offering a simple rule for telling woodpeckers apart by how often they call. Then it slips. The last category, "never: an ivory-billed," is a joke with a sting in it, because you can't identify a bird that never calls. The ivory-billed woodpecker is famously rare, possibly gone altogether, so the rule quietly admits a loss.
From there the tone turns wistful. "Each year the ground grows simpler / and the sky more complex" suggests that the land is losing its variety while something else, perhaps the open air, becomes harder to read. The reader has to decide what that means, and the poem doesn't spell it out.
The final image is the most direct. A hive of wild bees "used to sleep" in a hollow locust tree, and that past tense carries the feeling. The speaker doesn't complain loudly. He points at an empty spot and lets the absence speak. The mood is calm, observant, and gently sad.