The archaic vocabulary Bryant uses has two immediate effects, and several that follow. The first is defamiliarization. He's talking to and about a duck or similar bird, after all. Most people see them and move on. Using strange language draws attention to them. The closely related second effect is that the vocabulary slows the reader down. Words like "whither" and "illimitable" simply stall us, slowing readers down to spend more time on the scene. After that, the vocabulary elevates the bird, taking it from a forgettable duck to a kind of nature spirit.