In "To a Skylark," our speaker wants to know and say everything about this theme. He wants to know the secrets of nature, to know what birds feel when they sing. He's fascinated by the way that nature creates feelings in him, and by all the ways in which human beings interact with the natural world. This isn't just simple, cheery "look at the pretty birds" nature poetry. It's powerful, emotional, raw, and deep—a killer mix of human feeling and natural forces.
The poem's speaker works constantly to break down the barriers between humans and nature, to convince us that we are no different from the trees and flowers and skylarks all around us. Far out.
Finally, "To a Skylark" reveals the distance between mankind and the world. No matter how hard we try, we can never really understand the world around us. Not even with a GPS.