This poem falls under the genre of the "pastoral," which means that it celebrates natural beauty. But, the speaker doesn’t just celebrate; he also uses the natural world as evidence against people who say that "general ideas" are more beautiful than particular things. He argues that the source of all desire is a search for particular things we have loved and lost, like the peaceful riverbank of his childhood. Poetry, he thinks, is the best way to connect man’s intellectual world with the singular mysteries of nature. Even the word "blackberry" can be like a poem in praise of nature.
The poet uses the image of the "tragic" woodpecker to show the absurdity of the Platonic philosophy. It’s laughable to think that the woodpecker could relate in any way to our human ideas of tragedy.
Words are the link between the natural world of particulars and the intellectual world of ideas.