The least tern is North America's smallest tern. It measures 9 inches (23 cm) long and has a 20-inch (51 cm) wingspread. The bird's size and white forehead on its otherwise black cap are two of its best distinguishing characteristics. Other identifying marks are its black-tipped yellow bill, gray upperparts, white underparts and a short, forked tail. Weighing only an ounce (28 grams), this bird's appearance and habits have given rise to many nicknames, including "little striker" and "sea swallow." Sexes look identical, although females are slightly smaller. Juveniles have a black bill, a grayish crown and a black line of feathers running back from the eye.
Least Tern Nesting Habitat
The least tern breeds in three regions of North America; along the Pacific Coast, along the Atlantic and Gulf Coasts, and along several midwestern river systems in the interior. South Dakota's least terns are part of the "interior" population . Some experts consider the interior least tern to be a subspecies , Sterna antillarum athalassos , distinguishable from the coastal tern populations. This status is not accepted by others. Because of this debate among professional biologists, the interior least terns are listed as an endangered population, rather than the more formal designation of endangered subspecies .
Least terns usually arrive in South Dakota in early May. They select barren beaches along sandy or gravelly river shorelines or islands. Least terns have also adapted to artificial sites and sometimes nest in sand or gravel pits along rivers. Their small nesting colonies are close to feeding areas and may include another rare species, the piping plover. Male least terns court females near their selected nest site, where they scrape a shallow depression. Two or three brown speckled eggs are laid and incubated by both parents for about three weeks. Least tern chicks fledge about three weeks after hatching, although some parental care continues until the birds migrate south later in the summer.
Least terns feed almost entirely on small fish and aquatic crustaceans , which they capture by either of two main methods. They may fly low over water and snatch fish near the surface or hover high above to plunge headlong into the water to seize their prey. Their unusual nicknames are based on these habits.
Even though its breeding habitat can change dramatically from one year to the next, the least tern is considered to be quite faithful to its nesting areas. The majority of least terns live less than five years, although one bird lived more than 21 years.
Figure 1: Pair of Least Terns With Two Chicks: Male Feeding, Female Brooding.
The least tern was once abundant. During their Missouri River expeditions, Lewis and Clark commonly observed them. Near the end of the 19th century, least terns were slaughtered for the millinery trade. One report describes the annihilation of 1200 least terns in one day in Virginia. During the "Gay Nineties," least tern skins brought market hunters 10 to 12 cents each, to be mounted in one piece on a woman's hat.
Subsequent protection helped the least tern rebound by the 1920s. But
as our Midwestern rivers were harnessed, dammed,
channelized, and diverted, the least tern again suffered. Loss of nesting and possibly wintering habitat led to the 1985 listing of the interior population of the least tern as endangered , or in danger of extinction. The least tern's story is an example of the wildlife impact that can result from environmental modifications.
Because of the many threats to its nesting success, the least tern is
a true conservation challenge. In addition to the effects of
permanent river system changes, the least tern is vulnerable to water level fluctuations during the nesting season and loss of habitat because of vegetation growth. Disturbance of nesting sites by predators, people, pets, and livestock all negatively impact least tern populations.
In South Dakota, efforts have concentrated on careful inventories of
nesting colonies to evaluate likely threats. The U.S. Army
Corps of Engineers' Missouri River operations now place greater emphasis on avoiding losses of least terns and their nesting
habitats. Methods continue to be developed and refined to deter predators, people, and pets from nesting areas. Fences, law
enforcement patrols, warning signs, predator lights and removal, and public information efforts have all been used.
Many new techniques seek to mimic what our rivers previously did naturally. New nesting habitat is being created with material dredged from the river. Current habitat is being sustained by the removal of vegetation by mechanical means and through the use of pesticides. Such intensive management is needed to maintain the bare and open landscape least terns must have for nesting.
Crustaceans - the group of mostly aquatic arthropods that includes
lobsters, shrimp, crabs, crayfish, and wood lice.
Fledge - to become capable of flight and independent of parental care.
Population - a group of interbreeding plants or animals.
Subspecies - a geographically isolated and distinguishable segment of a species whose individuals will interbreed with other subspecies of the same species if ranges overlap.
Endangered - those animals or plants that are in danger of extinction throughout all or a significant part of their range and are listed for protection under the Endangered Species Act of 1973.
Millinery - the business of making women's hats.
South Dakota Ornithologists' Union. 1991. The Birds of South Dakota.
Second Edition. NSU Press, Aberdeen, SD.
Terres, J.K. 1980. The Audubon Society Encyclopedia of North American Birds. Alfred A. Knopf, New York.
U.S. Fish and Wildlife Service. 1990. Recovery Plan for the Interior Population of the Least Tern (Sterna antillarum ). U.S. Fish and Wildlife Service, Twin Cities, Minnesota.
Wetmore, A. 1965. Water, Prey and Game Birds of North America. National Geographic Society, Washington, D.C.
Selected Resources for Teachers
Fragile Legacy - Endangered , Threatened and Rare Animals of
South Dakota by D.E. Ashton and E.M. Dowd. 1991. S.D. Dept. of Game, Fish
and Parks, Wildlife Division Report. No. 91-04.
South Dakota Outdoor Guide Segment 205, featuring a story on the least tern and piping plover in South Dakota. Produced in 1989, this video is available for loan from the South Dakota State Library, Title No. 02774.
Eileen Dowd Stukel, S.D. Dept. of Game, Fish and Parks, Pierre, SD, 57501. 1994.
Julie Zickefoose, Route 1 Box 270, Whipple, Ohio 45788.
Dr. Dan Tallman, Northern State University, Aberdeen, SD 57401.
Publication of the Least Tern fact sheet was funded by the South
Dakota Department of Game, Fish and Parks, Division of
Wildlife, Pierre, SD.