Correspondents are various and include ornithologists, conservationists, conservation organisations (UK and abroad), colleagues and associates from Nicholson's years in the Civil Service and PEP, and others connected with Nicholson's activities and interests. The correspondence is largely personal and some covers matters of biographical interest. Ornithology is the principal topic in the 1920s and 1930s, continuing through the following decades; Nature Conservancy business is prominent during the 1950s and 1960s. Topics such as economics and Government administration which Nicholson covered in his book The System are also represented, while a few letters relate to his wartime employment.

The last file in the series is a reference for the following correspondence.