[1] From 1952, as a curatorial Assistant, he was assigned to the avian osteology collection of the British Museum (Natural History).
From 1971, he transferred the bird collections to the Natural History Museum at Tring where they were curated and supervised by Colston and Cowles.
In 1994, Cowles described the Réunion sheldgoose[5] (Alopochen kervazoi) and the Réunion kestrel (Falco duboisi) after extensive research on subfossil bird species of the Mascarene Islands in 1987 and in chapter "2-The Fossil Record" in the work Studies of Mascarene Island Birds.
During study of the Vergoz caverns near La-Saline-les-Bains and a coastal cave near Saint-Paul, he collected fragments of the extinct turtle species Saddle-backed Mauritius giant tortoise (Cylindraspis inepta).
[citation needed] Cowles published the book Birds (Instructions for Collectors) in 1970 in collaboration with Colin James Oliver Harrison (1926–2003).