 As a published expert on bird song, Hartshorne is the first philosopher since Aristotle to be an expert in both metaphysics and ornithology. He writes specifically of the aesthetic categories required to explain why birds sing outside of mating season and when territory is not threatened—two occasions for bird song that are crucial to the behaviorists’ account. Birds like to sing, he concludes. His discovery of the “monotony threshold” regarding bird song is still cited by ornithologists. Hartshorne’s ornithology thus serves to highlight the crucial role that aesthetic categories play in Hartshorne’s philosophy (from the ancient Greek word for feeling: aesthesis), from the minuscule feelings of microscopic reality to divine feelings.