Least Tern

Sterna antillarum

Description

Least terns are the smallest of the terns, measuring 8-9 in (20-22 cm) in length and having a 20 in (50 cm) wingspread. In breeding plumage both sexes have a black cap and nape, white forehead, grayish back and wings, snowy white underparts, orange legs, and a black-tipped orange-yellow bill. Immature birds have darker plumage, a dark bill, and dark eye stripes on their white heads. There are no consistent morphological, behavioral, or vocal differences between the least tern (Sterna antillarum antillarum ) and the California least tern (S. a. browni ). Only the interior population of least terns is endangered.

Behavior

Least terns live and breed in colonies. The nest is a simple unlined scrape, usually containing three brown-spotted, buff-colored eggs. Breeding colonies (terneries) usually consist of about 20 widely spaced nests. However, colonies of up to 75 nests have been reported on the Mississippi River. Egg-laying and incubation occur from late May to early August. Eggs are incubated for 20 days, and chicks fledge in another 20 days. Least terns capture small fish and minnows with a headfirst dive into the water.

Habitat

Least terns inhabit both coastal areas and interior river systems. Habitat requirements for interior least terns center around three ecological factors: presence of bare or nearly bare alluvial islands or sandbars, favorable water levels during the nesting season, and food availability. Under natural river conditions, islands are created and destroyed by the river's erosion and deposition processes. Periodic inundation keeps some islands barren or sparsely vegetated. Although most nesting is in rivers, the least tern also nests on the barren flats of saline lakes and ponds such as the Salt Plains National Wildlife Refuge, Oklahoma.

Distribution

The interior least tern historically bred along all the major inland river systems: the Colorado (in Texas), Red, Rio Grande, Arkansas, Missouri, Ohio, and Mississippi Rivers. The wintering area for this population is unknown; however, least terns of un-determined populations winter along the Central American coast and the northern coast of South America from Venezuela to northeastern Brazil. The least tern is not considered endangered when found within 50 mi (80 km) of the coast. The eastern least tern population breeds along the Atlantic coast from Maine to Florida and along the Gulf coast from Florida to Texas and probably winters in the Bahamas and Caribbean Islands. Formerly a common breeding bird throughout most of its current range, the interior least tern is now extremely limited in both numbers and distribution. Once found throughout the Mississippi and Red River valleys of Louisiana, it is now absent from that state. By 1990, census data indicated the existence of about 5,000 interior least terns.

In the Dakotas, the interior least terns occur on the segments of the Missouri River and its tributaries not affected by impoundments or channelization. In South Dakota, the interior tern nests primarily in flowing segments of the Missouri River and Cheyenne River. Breeding areas in North Dakota constitute about 119.3 mi (192 km) of the Missouri River from Garrison Dam to the mouth of the Cannonball River south of Bismarck and about 18 mi (29 km) of the Yellowstone River in North Dakota from the Montana border to the river's confluence with the Missouri River. A few interior terns nest on islands, shorelines, and sandbars along the reservoir Lake Oahe, an impoundment on the Missouri River in North and South Dakota. In Montana, breeding interior least terns have been recorded on the Yellowstone River and on the Missouri River between Fort Peck Reservoir and North Dakota. A few interior least terns have been recorded on islands and shoreline within the Fort Peck Reservoir. These locations are the westernmost nesting sites of the interior least tern. Interior least terns breed along the lower section of the Niobrara River, Nebraska, from Keya Paha and Rock Counties to the Missouri River. Current distribution is probably similar to historic distribution because the Niobrara River has been little changed by man. On the Platte River in Nebraska, the terns nest on sandbars and at sand and gravel pits from the Missouri River to North Platte and along the South Platte River as far west as Ogallala. On the Loup River, a tributary of the Platte, the terns breed as far west as Arcadia but are most common between Saint Paul, Nebraska and the Loup's confluence with the Platte River at Columbus, Nebraska. A few interior least terns also occur along the Elkhorn River, another Platte tributary.

Along the Arkansas River System, breeding interior least terns occur in Colorado, Kansas, Arkansas and Texas. In Colorado, the terns nest at Adobe Creek reservoir (Blue Lake) and have been observed at Nee Noshe reservoir. Both are located on small tributaries of the Arkansas River. In Kansas, the terns nest on the Cimarron River in Meade, Comanche and Clark counties, and Quivera National Wildlife Refuge; in the recent past, they have also been sighted at the Cheyenne Bottoms Wildlife Management Area.

The tern occurs on several tributaries of the Arkansas River in Oklahoma. It breeds along the Salt Fork of the Arkansas River at the Salt Plains National Wildlife Refuge; at the Optima Reservoir at the fork of the Coldwater Creek and Beaver River in the Oklahoma Panhandle; and on the Cimarron River in Beaver, Harper, Woods, Woodward, Blaine, Kingfisher, Logan and Payne counties. Along the Arkansas River in Oklahoma, the tern breeds in Kay, Osage, Pawnee, Creek, Tulsa, Wagoner, Muskogee, and Sequoyah counties. In Arkansas, the breeding range on the Arkansas River is above Little Rock. Along the Canadian River, interior least terns breed in Ellis, Roger, Mills, Dewey, Cleveland, McClain, Haskell, and Sequoyah Counties in Oklahoma and in Hemphill, Roberts, and Hutchinson Counties in Texas. On the Mississippi River, the terns occur almost entirely in the lower valley south of Cairo, Illinois to Vicksburg, Missouri. Surveys by the U.S. Army Corps of Engineers and the Missouri Department of Conservation during the late 1980s and 1990 indicate that about one-half of all interior least terns occur along a 683.5 mi (1100 km) stretch of the Lower Mississippi River. On the Ohio River system, the tern occurs just above the confluence of the Tennessee and Ohio Rivers, and at one artificial site on the Wabash River in Indiana.

The birds are also known to occur on the Prairie Dog Town Fork in the eastern Texas Panhandle and along the Texas/Oklahoma boundary as far east as Burkburnett, Texas (in the Red River System); and three reservoirs along the Rio Grande River and Pecos River at the Bitter Lake National Wildlife Refuge in New Mexico (in the Rio Grande system).

Threats

Nesting habitat throughout the central and western United States has been altered by increased vegetation brought about by man-made changes in river flow. Maintenance of sandbar habitat will aid recovery, and although least terns prefer natural islands or sandbars, they will nest on man-made sites on river floodplains. Such sites, however, are usually connected to the shore and accessible to predators and human disturbance.

Conservation and Recovery

Among the many attempts to protect least terns, the Oklahoma Department of Wildlife Conservation and the U.S. Fish and Wildlife Service (FWS) have agreed on a recovery effort for least terns in Oklahoma. The joint project will focus on increasing public awareness of the least tern through television and newspaper coverage of the breeding season.

The 1990 Recovery Plan for the Interior Population of the Least Tern, from the FWS, notes that the recovery objective is delisting of the population, perhaps as early as 2005 if recovery criteria are met. Before the population can be delisted, the protection of the essential habitat must be assured by removal of current threats and habitat enhancement. Agreed upon management plans must be established, and a total population of 7,000 adult interior least terns must be stabilized for ten years in the bird's traditional habitat areas (2,100 in the Missouri River system; 2,200-2,500 on the Lower Mississippi River; 1,600 in the Arkansas River system; 300 in the Red River system; and 500 in the Rio Grand system).

To achieve the recovery goals, the FWS set forth a variety of necessary actions, including the determination of population trends and habitat requirements; the protection, enhancement and increase of populations during breeding; and the management of reservoir and river water levels to the benefit of the species. The plan also calls for the development of public awareness and implementation of educational programs, and for the implementation of law enforcement actions at nesting areas in conflict with high public use.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360
http://midwest.fws.gov/

References

Downing, R. L. 1980. "Survey of Interior Least Tern Nesting Populations." American Birds 34 (2): 209-211.

Gochfeld, M. 1983. "Colony Site Selection by Least Terns: Physical Attributes of Sites." Colonial Waterbirds 6: 205-213.

Schulenberg, J. H., and M. B. Ptacek. 1984. "Status of the Interior Least Tern in Kansas." American Birds 38 (6): 975-981.

U.S. Fish and Wildlife Service. 1990. "Recovery Plan for the Interior Population of the Least Tern." U.S. Fish and Wildlife Service, Twin Cities, Minnesota.