In the same year he became a research student at the Edward Grey Institute of Field Ornithology (EGI) and accompanied the scientists' couple Bernard and Sally Stonehouse and the ornithologist Doug Dorward on a two-year expedition of the British Ornithologists' Union to Ascension Island in the South Atlantic.
Through the mediation of David Lack, who worked with G. Evelyn Hutchinson at the EGI, Ashmole received a summer research fellowship at the Peabody Museum of Yale University.
Subsequently, the Ashmoles spent a year in Hawaii on a Yale fellowship at the Bernice P. Bishop Museum from where they studied feeding ecology and breeding cycles of terns and other seabirds on Kiritimati, as well as trying to assess the effects of nuclear weapons testing on sea birds.
Subsequently, Ashmole served as assistant and associate professor at Yale University, where he did research work until 1972.
[5] Ashmole's work on Ascension Island led him to propose a hypothesis about how large concentrations of seabirds might be able to deplete forage fish resources in the vicinity of their breeding colonies, creating a zone of reduced food availability that would influence foraging and breeding success and behaviour.