This series presents an alphabetical sequence of Nicholson's correspondence with friends and key colleagues and associates (C.P. Blacker, Jacob Bronowski, H.J. Coolidge, C.S. Elton, J-P Harroy, C.W. Hurcomb, Sir Julian Huxley, Ludwig Koch, Nicholas Polunin, Sir Arthur Tansley, and E.B. Worthington). Nicholson's letters of condolence, including to widows, and obituaries by him are sometimes included.

There is a small group of Nicholson's selected ornithological correspondence, 1928-1998. Stanley Cramp, P.A.D. Hollom, V.C. Wynne-Edwards and Phyllis Barclay-Smith are some of the correspondents.

The series is completed by a sequence of general correspondence, 1920-2002, chiefly personal and covering all aspects of his working life. R.E. Boote, R.S.R. Fitter, P.A.D. Hollom and Aubrey Buxton are some of the principal correspondents.