We rediscovered Rex's AOU (American Ornithologists Union) checklist in the summer of 2025.
Rex's handbound and heavily annotated checklist is based on the first edition of The Code of Nomenclature and Check-List of North American Birds, originally published in 1886. Though subsequent editions were published in 1895 (second edition), 1910 (third edition), and 1931 (fourth edition), Rex's collection of paintings was based on a reference that was twenty-four years out of date when he sat down to paint his final set of paintings.