The Caspian Tern in Washington State originally nested only in eastern Washington, but now more than 80% breed in Grays Harbor in western Washington. The shifting of breeding sites in Grays Harbor during the last ten years is believed to be related to competition and predation from gulls and erosion of nesting islands. A major threat to breeding success in small colonies occurs during the incubation period when human disturbance allows gulls access to eggs. Large colonies are most affected by human disturbances when chicks run from their territories and enter the territories of other aggressive adults. Washington State now represents the northern limit of the breeding range of the Caspian Tern (Sterna caspia) on the Pacific Coast (A.O.U. 1957). Breeding colonies were established in eastern Washington in the 1930's and in western Washington in Grays Harbor in 1957 (Alcorn 1958). Since the terns' appearance in Grays Harbor, the breeding population has increased steadily, while individual colonies have undergone dramatic population fluctuations. The extension of the tern's breeding range is continuing to spread northward into British Columbia as evidenced by increasing numbers of summer sightings and young of the year being found there (R. W. Campbell, pers. comm.). Recent range extension and rapid shifts of breeding locations in Washington demon- strate that the occurrence of the Caspian Tern in the Pacific Northwest is characterized by change rather than stability, a characteristic shared by other larid species at other locations (e.g. Ludwig 1966, 1974; Kadlec and Drury 1968; Nisbet 1973). It therefore seems appropriate to summarize the present known distribution of the species in Wash- ington so that future change can be properly evaluated. METHODS