Rare Species Guide

  Basis for Listing

The breeding range of the Forster's tern extends across the interior of the western United States and Canadian Prairie Provinces, with outlier populations in the southern Great Lakes, mid-Atlantic Coast, and western Gulf Coast. The Northern Great Plains, including western Minnesota, comprise the species' primary range. In Minnesota, the Forster's tern is found throughout the western prairies and eastward through the prairie-woods transition, including an extension into the central part of the state. The range expanded eastward towards the Twin Cities metropolitan area in the last 50 years.

Although the range of the Forster's tern covers at least one third of Minnesota, the species does not occur as commonly on prairie marshes as it did 50 years ago. Approximately 50 active colonies have been documented since 1990, but only 11 of these included either an adult population of at least 100 birds or more than 50 nests. Furthermore, much apparently suitable habitat in the state is not being utilized. Studies in 1985 and 1986 found most breeding terns were limited to just four colonies in central Minnesota and one colony in northwest Minnesota (Louis 1989). Based on the breeding population estimates from this study, Cuthbert and Louis (1993) reported a population decline of approximately 60% since 1942. Given population declines and the continued loss and degradation of the species' prairie marsh habitat, the Forster's tern was listed as a special concern species in Minnesota in 1984.

  Description

The Forster's tern is a slender, gull-like bird with a black cap, pointed wings, forked tail, gray back and wings, and white underparts. This species is very similar in appearance to the common tern (Sterna hirundo), another rare species. During the breeding season, the end of the upperwing of the Forster's tern is largely white, paler than the rest of the wing, and the underparts are white. Common terns differ in that the upperwing has a dark wedge near the tip, and the underparts are grayish. Subtle differences between these species include the bill and leg color, which is orange in the Forster's tern, as opposed to reddish in the common tern. Forster's terns can also be distinguished by their calls, which are somewhat lower and shorter in duration than those of the common tern.

  Habitat

During the breeding season, Forster's terns prefer extensive marshes with an interspersion of emergent vegetation and open water. Floating platforms of vegetation or muskrat (Ondatra zibethicus) houses are selected for nesting sites. Water level appears to be a crucial factor in selecting suitable nesting habitat as well as for overall nesting success. Forster's terns prefer deeper, open portions of marshes (McNicholl et al. 2001). Previous nesting sites that have unsuitable water levels when the birds return in the spring may be quickly abandoned in favor of other more appropriate sites. This flexibility allows the species to adapt more readily to unreliable water conditions.

  Biology / Life History

The Forster's tern is a short- to medium-range migrant, with inland breeding populations moving to marshes, lakes, or beaches near the southern Atlantic, Pacific, and Gulf coasts of North America in winter. Some populations breed and winter in the same southern coastal areas (McNicholl et al. 2001). Forster's terns nest in colonies, and vigorously defend the nest area when incubating their eggs. Colony size ranges from about 2-100 nests, although colonies in the thousands have been documented. Nests are built on muskrat lodges or floating vegetation, such as bulrushes, grasses, cattails, or sedges. They are usually close to open water. In coastal areas, Forster's terns may scrape the ground to create a nest (McNicholl et al. 2001). Clutch size varies from 2-3 eggs. Incubation occurs for 20-28 days, with both adults incubating the eggs. Young are semi-precocial, and leave the nest after about 4 days. Fledglings are able to fly after 4-5 weeks. Adults feed young during both nestling and fledgling stages, and may even continue to feed juveniles during migration. Eggs and young are vulnerable to weather and nest predators. But after flood events, adult females appear able to lay a second clutch quickly (McNicholl et al. 2001).

The main food of the Forster's tern is small fish. Forster's terns forage in both freshwater (marshes and lakes) and saltwater (estuaries and coastal zones) by looking down while flying over water, then plunging to nab prey. A bird may become partially or completely submerged during a plunge (McNicholl et al. 2001).

  Conservation / Management

The Forster's tern is vulnerable to human disturbance, water level fluctuation, chemical contamination, nest predation, and habitat destruction. Motorboats passing near a colony cause Forster's terns to fly from nests (Fraser 1994), potentially exposing eggs or young to inclement weather or predation. Frequent human disturbance may cause colony abandonment, although Forster's terns may also become habituated to boat traffic. Water fluctuation caused by storms, boat waves, or high tides can destroy eggs, young, or nesting habitat. Chemical contamination resulting in high levels of organochlorines and heavy metals in eggs or adults may cause reproductive problems. For example, in Green Bay, Wisconsin high levels of PCB were associated with abnormal embryos and reduced hatching success of Forster's terns (McNicholl et al. 2001). Nest predation by great horned owls (Bubo virginianus) and mink (Mustela vison) can also cause Forster's terns to abandon nesting colonies (Fraser 1994; McNicholl et al. 2001). Forster's terns can benefit from preservation and restoration of marsh habitats. The species has been shown to colonize restored areas, and to use wooden platforms for nesting (McNicholl et al. 2001).

  Conservation Efforts in Minnesota

In 1985 and 1986, researchers visited all the major Forster's tern colonies throughout the state and found that the bulk of the breeding population was restricted to a few large colonies in Marshall, Nicollet, Jackson, Todd, and Wright counties. Most colonies in the northwestern quarter of the state presumed to support larger breeding populations were found to have only a small number of nesting birds. Based on this research, the total breeding population of Forster's terns in Minnesota was estimated at 900-1,000 pairs, significantly lower than the largest previous estimate of 2,500 pairs in 1942 (Louis 1989). In 1990, a survey of Forster's terns was conducted in the Minnesota Valley Wildlife Refuge in the Twin Cities area. Although Forster's terns were seen on a variety of lakes within the refuge, only seven nests in two wetland areas were found, and no young successfully fledged. Flooding and predators appeared to be responsible for nest failures (Dulin 1990). Studies of Forster's tern colonies on Lake Osakis in Todd and Douglas counties from 1992-93 showed very low reproductive success, even in a colony of 132-158 pairs. Mink predation and inclement weather both played a role in the reproductive failure (Fraser 1994). A thorough survey to locate nesting colonies throughout the state is needed, followed by periodic monitoring to determine their occupancy, size, and productivity. Sources of disturbance to known breeding areas should be identified, and minimized or eliminated if possible. Suitable Forster's tern habitat should also be protected or restored, especially near established colonies. Natural features common to successful breeding colonies should be identified and incorporated into restoration projects.

  References and Additional Information

Cuthbert, F. J., and M. Y. Louis. 1993. The Forster's Tern in Minnesota: status, distribution, and reproductive success. Wilson Bulletin 105(1):184-187.

Dulin, G. S. 1990. 1990 summary report of the Forster's and Black Tern breeding survey of the Minnesota Valley National Wildlife Refuge and vicinity. Report submitted to the Minnesota Valley National Wildlife Refuge and the Nongame Wildlife Program, Minnesota Department of Natural Resources. 16 pp.

Fraser, G. 1994. Feeding ecology and nesting success of Forster's Terns on lake Osakis, Minnesota. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 35 pp.

Louis, M. Y. C. 1989. The Forster's Tern in Minnesota: status, distribution, and reproductive success. M.S. Thesis, University of Minnesota, Minneapolis, Minnesota. 33 pp.

McNicholl, M. K., P. E. Lowther, and J. A. Hall. 2001. Forster's Tern (Sterna forsteri). Number 595 in A. Poole and F. Gill, editors. The birds of North America. The Birds of North America, Inc., Philadelphia, Pennsylvania.