Career
From 1894-1895 he worked at the Field Museum of Natural History as assistant curator of zoology where, despite his speciality in ichthyology, he worked in all non-ornithological fields of zoology. In 1912, Hay was appointed as a research associate at the Carnegie Institution for Science, and was given office space at the United States National Museum. There he did much work with the USNM"s collections in vertebrate paleontology.
He published extensively on fossil turtles and Pleistocene mammals.
The catalogs that he constructed were a great aid in recording existing knowledge and became standard references. His papers from 1911 to 1930 are stored at the Smithsonian Institution.