Simplify the following text for a Grade 9 native English-speaking person.
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 .