花鳥風月 (Ka-Chou-Fuu-Getsu), the title of the OP song, means Flower-Bird-Wind-Moon, the wonders of Nature. If you pay attention during the opening, as Ka-chou-fuu-getsu is sung, characters are associated with each word. 1st: Yukari (flower), Yuyuko (bird), Kanako (wind) and Eirin (moon) 2nd: Reimu (flower), Marisa (bird), Sanae (wind) and Sakuya (moon). They surely thought this carefully, and I especially find the second association very matching.