Sterna antillarum athallasos
DESCRIPTION: The least tern is about nine inches long with a wing span of 20 inches. Bent (1921) referred to the least tern as the "sea swallow" for its rapid, darting flight. Breeding plumage for this tern consists of a black cap, white forehead, throat and underside with a pale gray back and wings, and black-tipped yellow-orange bill. In flight, the least tern is distinguished by the long, black outermost wing feathers and the short, deeply forked tail. First-year birds have a dark bill, a dark gray eye stripe, and a dusky brown cap.
STATUS: The California and interior populations of the least tern were federally listed as endangered in 1970 and 1985, respectively (Federal Register 35:16047-16048 and 50:21784-21792). The eastern least tern is not federally listed, but is protected by state laws as a threatened or endangered species.
REASONS FOR CURRENT STATUS: Lewis and Clark's frequently observed least terns along the Missouri River during their 1804 expedition. In the past century, the number of Least Terns has fluctuated widely. During the late 1800s, Least Terns declined in numbers due to harvesting for the millinery trade. After the Migratory Bird Treaty Act was passed in 1916 to make commercial harvest illegal, tern numbers increased until the mid-1900s when alterations to hydrologic patterns, and urban and industrial development of shorelines led to further population declines. Reduced flooding prevents scouring of sandy islands and shores, allowing vegetation to grow and making the habitat unsuitable for nesting terns. River channelization, gravel mining and human-related disturbance (i.e., foot traffic, unleashed pets, swimmers, canoeists and off-road vehicles) have all contributed to the decline of this species. Indirect disturbance of least tern colonies can result in temporary abandonment of nests (Burger 1981), exposing adults to aerial predation and eggs and chicks to predation and inclement environmental conditions.
A few stretches of the Missouri River below Ft. Randall and Gavins Point dams are the only river segments in South Dakota that still contain naturally occurring sandbar nesting habitat for least terns. However, habitat availability and reproductive success of interior least terns are detrimentally impacted by discharge of water for hydroelectric power and navigation that flood nesting islands.
REPRODUCTION AND DEVELOPMENT: Least Terns are long-distance migrants that breed in North America and winter in South America. In South Dakota, they typically arrive on their nesting grounds in early to mid-May. Interior least terns form loose colonies of 3 - 30 pairs. Predators or other intruders are dive-bombed and defecated on by adults. The nest cup is little more then a shallow scrape. Interior least terns usually lay two to three eggs per nest and may renest if their nest is destroyed. A single brood is raised each year and adults continue to feed offspring for several weeks after fledging. Chick survival varies substantially between sites and years.
Incubation period ranges from 19-25 days. Although the female does most of the incubation and brooding, both adults participate. Chick color varies from white to tan with black spots or streaks across back and top of head. Chicks quickly become mobile and leave the nest within a few days of hatching. Chicks are fed small minnow-like fish until they fledge at around 20 days. Recently fledged chicks are inefficient predators and continue to receive food from adults for several weeks. Fledglings may disperse from natal colonies within 3 weeks of fledging.
RANGE: Three subspecies of least tern nest in North America. The California least tern (Sterna a. brownii) nests from Baja California to the San Francisco Bay; the interior least tern (Sterna a. athalassos) nests along the major tributaries throughout the interior U.S. from Montana to Texas and New Mexico to Louisiana; and the eastern least tern (Sterna a. antillarum) nests along the coast from Texas to Maine.
POPULATION LEVEL: Total population estimates have been difficult to estimate because least tern colonies are small and transitory. The interior population was estimated at about 7,000 individuals around 1990.
HABITAT: Interior least terns nest on open shorelines, riverine sandbars, and mudflats throughout the Mississippi and Missouri river drainages. Suitable nesting habitat is sparsely vegetated with sand or gravel substrate and located near an adequate food supply. Site tenacity is strongly influenced by the dynamic nature of river hydrology which may change island size and vegetative cover annually.
MANAGEMENT AND PROTECTION: Least tern populations continue to be threatened by habitat loss and human disturbance. On their wintering grounds in South America, they are killed for food or sport while eggs are collected in the Caribbean for food. Local management efforts provide fences or signs to mark nesting sites as off-limits to recreation, structures (i.e., electric fences, chick shelters) to exclude predators, creation of artificial nesting sites, and habitat improvements. River flows also are being managed in some areas to protect nesting sites. Because least terns are sensitive to human disturbance, a buffer zone of at least 1/4 mile around active nesting colonies is recommended for any recreation or construction activities.
Information summarized from:
Bent, A. C. 1921. Life histories of North American gulls and terns. U.S. National Museum Bulletin 113:270-279.
Burger, J. 1981. Effects of human disturbance on colonial species, particularly gulls. Colonial Waterbirds 4:28-36.
Nebraska Game and Parks Division. The interior least tern an endangered species. http://www.ngpc.state.ne.us/wildlife/ltern.html
Thompson, B. C., J. A. Jackson, J. Burger, L. A. Hill, E. M. Kirsch, and J. L. Atwood. 1997. Least Tern (Sterna antillarum). In The Birds of North America, No. 290 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.
USFWS Interior Least Tern Species Profile page
Last updated: May 3, 2011