The interior population of least tern was listed as endangered in 1985. Populations along the East and West coasts are not endangered. The interior population, which once inhabited all the major river systems in the middle of the country, evolved to take advantage of constantly changing rivers and fish concentrated in pools by retreating summer flows. When river impoundment, channelization, and levee construction altered natural flow regimes and prevented flow into oxbows and backwaters, least terns rapidly declined.
Following their listing as endangered, least terns were first documented in Montana at Fort Peck Reservoir in 1987. To date, almost all the least terns in Montana have been found in three areas: the eastern end of Fort Peck Reservoir above Fort Peck Dam along the Big Dry Arm; the Missouri River below Fort Peck Dam; and the Yellowstone River below Miles City.
In 1994, eight nests were found at Fort Peck Reservoir, all on a small island within sight of Fort Peck Dam. The eight nests were initiated by six pairs of least terns and were near the nests of some common terns. Only three nests were successful and, of six young produced, only two birds fledged. Nest and chick loss was probably due to gull predation. This was the third year least tern production was observed at Fort Peck Reservoir, and the most nests seen since surveys began in 1987.
In the Missouri River below Fort Peck Dam, the U.S. Army Corps of Engineers is trying to minimize impacts on least terns by reducing flows during the May-through-August nesting period. In 1994, biologists surveying a 126-mile stretch of the Missouri below Fort Peck Dam found 23 least tern nests in five locations, all in one 31-mile reach. Fourteen of the nests were successful; of 28 young produced, at least 19 fledged. This is the largest number of least terns produced in the lower Missouri since surveys began in 1988.
In 1994 a study of least tern and piping plover reproduction was begun on the Yellowstone River between Miles City and Crane, Montana (143 miles). That year, 17 tern nests on nine islands in a 98-mile stretch produced 45 young. In all, 40 adults were observed.
Delisting the least tern can be considered when the interior population reaches 7,000 and is stable for 10 years, as verified by four censuses. The goal for the Missouri River system is 2,100 birds. Montana, which is at the western edge of the range, has a recovery goal of 50 birds. This was exceeded in 1989, 1990, 1991, 1992, and again in 1994, when a record 51 pairs produced 81 young.