During his spare time he studied and collected the birds of India. His books included "The Indian Ducks and their Allies" (1908), "Game Birds of India and Ceylon" (1921), "The Fauna of British India, Including Ceylon and Burma". "Birds". (1922; eight volumes), "Mishmi Man-eater" (1928), "The Nidification of the Birds of the Indian Empire" (1932), and "Cuckoo Problems" (1942; the cuckoo was his chief interest within ornithology). He made a comprehensive collection of nearly 50,000 Indian birds' eggs, part of which he donated to the Natural History Museum, where he spent a lot of time working on the egg collections from India and Thailand. His eight-volume contribution to "The Fauna of British India, Including Ceylon and Burma" series became the standard reference work on the subject. Part of the collection, about 152 specimens were sold to the private museum of the Tzar Ferdinand I of Bulgaria. He also served on government advisory committees on the protection of birds and was from 1913 to 1936 honorary secretary and treasurer of the British Ornithologists' Union.