Original: 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.
Simplified: White believed in observing birds rather than collecting specimens. He was one of the first people to separate the similar-looking Chiffchaff, Willow Warbler and Wood Warbler by means of their song.