Forster's Tern is the only tern restricted almost entirely to North America throughout the year. Its highest breeding numbers are found around south-central Manitoba, northern California—southern Oregon, and the Gulf Coast. Its centers of abundance in early winter match those of its coastal breeding areas, particularly around the Virginia–North Carolina border, Galveston Bay (Texas), and near Jacksonville, Florida. Similar in appearance and ecology to several other terns, it is primarily a marsh bird, although it is also found along Atlantic, Pacific, and Gulf coasts.
A “marsh tern,” this species breeds primarily in fresh, brackish, and saltwater marshes, including marshy borders of lakes, islands, or streams. It is found more often in open, deeper portions of marshes, generally in wetlands with considerable open water and large stands of island-like vegetation and/or large mats of floating vegetation. Along the Atlantic Coast, Forster's Tern breeds in marshy portions of beach and estuarine areas. The suitability of nesting habitat is often ephemeral, varying at a given site from year to year.
Major studies on Forster's Tern have focused on its breeding ecology in Manitoba (McNicholl McNicholl 1971, McNicholl 1980c, McNicholl 1982, McNicholl 1983), feeding ecology in Minnesota (Fraser Fraser 1994a, Fraser 1997) and at San Francisco Bay, CA (Salt and Willard 1971), and vocalizations and behavior in Washington (Hall Hall 1988a, Hall 1989, Hall 1998c).