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However, the validity of least tern subspecies has been questioned by several authors in recent years. Massey (1976) reported no consistent morphological, behavioral, or vocal differences between ~. ~. antillarum and ~. ~. brown!. In Texas, where both ~. ~. anti1larum and ~. ~. athalassos occur, electrophoretic analyses indicate little genetic differentiation between least terns produced on the Texas coast and Texas Panhandle rivers (McCament and Thompson 1987, McCament-Locknane 1988). Coastal least terns have populated interior breeding sites. Boyd and Thompson (1985) reported an incubating least tern at Quivira National Wildlife Refuge, Kansas, that originally had been banded as a chick on the Texas coast. The most recent morphometric and biochemical assessment of North American least terns could not distinguish subspecies (Thompson et a1. In prep) Originally, ~. ~. athalassos was proposed for endangered status. Because of the taxonomic uncertainty of least tern subspecies in North America, the U. S. Fish and Wildlife Service did not list the subspecies and instead designated as endangered those least terns occurring in interior North America. The California least tern has been listed as endangered since 1970 (U. S. Fish and Wildlife Service 1980). Distribution The interior least tern is migratory and historically bred along the Mississippi, Red and Rio Grande River systems and rivers of central Texas. The breeding range extended from Texas to Montana and from eastern Colorado and New Mexico to southern Indiana. It included the Red, Missouri, Arkansas, Mississippi, Ohio and Rio Grande River systems (American Ornithologists' Union 1957, Anderson 1971, Coues 1874, Burroughs 1961, Hardy 1957, Youngworth 1930, 1931, Ducey 1981). Incidental occurrences of least terns in Michigan, Minnesota, Wisconsin, Ohio and Arizona have been reported (Campbell. 1935, Janssen 1986, Jung 1935, Mayfield 1943, Monson and Phillips 1981, Phillips et al. 1964). Current Distribution The interior least tern continues to breed in most of the aforementioned river systems, although its distribution generally is restricted to less altered river segments (Figure 1) (Tables 1-5). Missouri ~ Svstem: The explorers, Lewis and Clark, observed the least terns along the Missouri River frequently and believed them to be "a native of this country and probably a constant resident" (Burroughs 1961). In the Dakotas, most interior least terns occur on those segments of the Missouri River and its tributaries that are not affected by impoundments or channelization. In South Dakota, the interior least tern nests primarily on flowing segments of the Missouri River and Cheyenne River (Nebraska Game and Parks Commission, Schwalbach 1988, Schwalbach et al. 1986, 1988). Breeding areas in North Dakota constitute about 192 km of the Missouri River from Garrison Dam to the mouth of the Cannonball River 3