In the late twenties Neville Cayley, a member of the Council for some years, was occupied in writing and illustrating a comprehensive guide to Australian birds. In return for the Council’s sponsorship he assigned four tenths of his 10% royalty to the League, and in 1931 at the 21st Bird Day celebration in the Assembly Hall, "What Bird is That?" was launched. Despite early doubts, What Bird? was a huge success; in 1960 it was rated the all-time best seller in Australian natural history books, and is still in print today. But initial sales were disappointing, and in February 1935 Cayley, perennially short of cash, offered to sell his remaining royalty share to the League for £300. With considerable misgivings, Council agreed. Sales took off during World War II, sparked interest from US servicemen in Australia, and the book turned out to be a lucrative investment.