Input (grade 13): He believed in distinguishing birds by observation rather than by collecting specimens, and was thus one of the first people to separate the similar-looking Chiffchaff, Willow Warbler and Wood Warbler by means of their song.
Output (grade 11): He was one of the first people to separate the similar-looking Chiffchaff, Willow Warbler and Wood Warbler by means of their song.