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 . 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 .