I know I've said this before, but the great thing about writing a comic with animals as the main characters is that you have so many opportunities to criticize humanity and completely get away with it. I think Lewis was really thinking of the superiority of bird eyesight, because they can see much better than we can. In fact, they can even see ultraviolet light! Of course, Clarence is astute enough to realize that the other definition of blindness is applicable as well, because people sure are stupid.