The fonds consists of two original draft manuscripts written by Thomas McIlwraith, the "Birds of Ontario" and the "Birds Observed in the Vicinity of Hamilton," as well as business postcards from Thomas McIlwraith's merchant coal business.

The material in the fonds consists of predominantly textual materials pertaining to the organization’s conservation projects, programming, public education, promotional materials and publications. The fonds contains eleven series and one subseries including, administrative records, correspondence, bird records, environmental conservation projects, programs and events, the Junior Audubon Club, property records, bequests, endowments and funds, awards and recognition, publicity and publications, photographs and scrapbooks. The fonds also contains graphic materials including water colours, as well as photographs of members, events, programs, wildlife and plant life.