"Thrushes" uses intense descriptions of the natural world to reflect on human nature. More specifically, it compares the clarity, intensity, and almost purity of animal action to the labor required for humans to act, especially human artists. Everything distracts the human artist, making it hard for him to do at all, and especially to create. The entire spiritual world seems aimed at distracting him, and the entire natural world (the waters) laments his situation.
Though it can be generalized more broadly, this could be read as addressing Hughes' own struggles as an artist.