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Fragile Legacy


Endangered, Threatened & Rare Animals of South Dakota


Interior Least Tern (Sterna antillarum athalassos)


JPG--species photo species distribution map
Status: Federal Endangered, State Endangered

Description: The interior least tern is the smallest member of the gull and tern family, measuring 8-9 inches (20-23 cm) long and having a 20 inch (51 cm) wingspread. Males and females appear identical with a black crown, white forehead, gray back, gray wings above with white below, orange legs and a black-tipped yellow bill. Immature birds have darker feathers, a dark bill and dark eye stripes on white heads.

Habitat and Habits: The interior least tern arrives on its breeding grounds in early May. It nests in small, loosely defined groups on barren beaches of sand, gravel or shells, on dry mudflats and salt-encrusted soils (salt flats) and at sand and gravel pits along rivers. Nesting success depends on the presence of bare or nearly barren sandbars, favorable water levels during nesting and abundant food.

The nest is an inconspicuous, unlined scrape usually containing three brown spotted eggs. Egg-laying and incubation occur from late May through early August. Eggs hatch in about 20 days and chicks are fledged in about another 20 days. The interior least tern feeds on small fish and crustaceans taken by diving from the air into shallow water. During the breeding season, these birds usually feed within a few hundred meters of the nesting colony.

Distribution: The interior least tern historically nested along the Colorado (in Texas), Red, Rio Grande, Arkansas, Missouri, Ohio and Mississippi River systems. It currently nests in the Mississippi and Rio Grande River basins from Montana south to Texas and from eastern New Mexico and Colorado to Indiana and Louisiana. This species is thought to overwinter in Central and South America. Approximately 400 least terns nest along the Cheyenne and Missouri Rivers in South Dakota, with the majority concentrated below the Gavins Point and Fort Randall Dams.

Conservation Measures: The interior least tern was federally listed as endangered in 1985, primarily due to the loss of nesting habitat as a result of dramatic alterations (channelization and impoundment) of important river systems. Water level fluctuations, vegetation of nesting habitat and disturbance (from people, pets, predators and livestock) continue to jeopardize nesting success.

Reproductive success along the Missouri River depends greatly on careful water level management and protection of nesting birds from disturbance. Several methods have been used to deter humans, pets and predators from nesting areas, including fences, warnings and public information efforts.

Major river systems are no longer allowed to follow the natural cycle of spring flooding, which previously resulted in the dynamic creation and movement of sandbar habitat. Many interior least tern management techniques mimic what rivers previously did naturally. Such techniques include the creation of new nesting habitat with material dredged from the river channel and the removal of vegetation from potential nesting habitat by hand, by machine, by chemical means or through a combination of techniques.


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