This series presents a comprehensive record of much of Nicholson's ornithological fieldwork, both organised and casual. The material largely comprises manuscript lists of bird sightings, many organised on a geographical basis, for example, England (by county, district), Wales, European countries, non-European countries. These range widely in date, from 1915 to the 1980s; a considerable quantity are from the 1930s, 1960s, and 1970s. There are a few papers from the 1928 Oxford Bird Census. These loose lists are augmented by a series of ten ring-binders, seven forming an alphabetical sequence: the greater portion of these lists date from the 1920s, though the coverage continues to the 1980s.

Nicholson's participation in the two Oxford University Exploration Club expeditions, to Greenland, 1928, and to British Guiana, 1930, is also represented in this section. The Greenland papers include drafts and notes on birds, and general observations, a draft talk to the Oxford Ornithological Society and several photographs annotated by Nicholson. Among the British Guiana material are four ornithological field notebooks, further notes and logs, drafts of at least three published papers and correspondence arising from the expedition. One much later expedition is also documented: Nicholson's ornithological survey of bird population and ecology in Baluchistan, Pakistan, 1952. Various field notes survive and there is also a draft of a paper on the survey given at a 1961 British Ornithologists' Union Conference.