Sterna hirundo Linnaeus, 1758
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Basis for Listing
The common tern is a holarctic species with an extensive range throughout Europe and Asia. In North America, it nests primarily in three areas: the northern and mid-Atlantic Coast, the Great Lakes, and the northern Great Plains. The Great Lakes population has been experiencing problems with predation, human disturbance, and competition for breeding sites with exploding ring-billed gull (Larus delawarensis) populations. The Great Lakes common tern population has declined to a point where the species is classified as threatened or endangered in most of the Great Lakes states. Minnesota's adult population of common terns numbered over 2,000 breeding pairs in 1900, but declined to 880 pairs by 1984. As a result, the common tern was classified as a species of special concern in Minnesota in 1984. Continuing declines in the state led to the species reclassification as threatened in 1996. Currently, there are less than 20 nesting common tern colonies in Minnesota, representing a statewide population of fewer than 900 nesting pairs.
The common tern is a slender, gull-like bird with a black cap and nape, gray back and wings, pale grayish underparts, pointed wings, and a forked tail. This species is very similar in appearance to the Forster's tern (Sterna forsteri), another rare species. Subtle differences between the two species include the bill and leg color, which is reddish in the common tern, and orange in the Forster's tern. Additionally, during the breeding season, common terns have a dark wedge near the tip of the upperwing and grayish underparts. Forster's terns lack this dark wedge, having largerly white upperwing tips that are paler than the rest of the wing, and the species has white underparts. Common terns can also be distinguished by their calls, which are somewhat higher and longer in duration than those of the Forster's tern.
Common terns select isolated, sparsely vegetated islands in large lakes for nesting. Open edges of sandy or gravelly beaches or dredge spoil areas are also used. Optimal breeding sites are isolated from predators by natural barriers, have a constant, nearby source of food, have stable or falling water levels during the nesting season, and have topography that allows nesting common terns to see and hear their neighbors.
Biology / Life History
Common terns nest in large colonies and invading diurnal predators are mobbed by all adults in a colony. Elsewhere in their range, other species, such as black skimmers (Rynchops niger), often nest in tern colonies, presumably because the cooperative mobbing of predators by the terns protects the skimmer nests as well. In Lake of the Woods in northern Minnesota, piping plovers (Charadrius melodus) and common terns have nested in proximity to one another. Common tern nests are shallow indentations in the sand or wracks of dead vegetation on beaches. An average clutch contains 3 eggs. Both parents incubate the eggs during the day, but at several island sites in Minnesota, the parents fly to the mainland during the night, leaving the eggs or chicks unguarded. Parents who incubate their nests during the night are especially vulnerable to predators, as they are on the ground and in the open. It is thought that night desertion of nests may be a way for parents to escape predation. However, it increases the incubation period of eggs (Nisbet 1975). Common tern chicks do not start to fly until approximately 21 days of age, but they usually leave the nest after several days and hop around on the beach or hide in vegetation (McKearnan 1986) while waiting for their parents to return with food. This species eats fish almost exclusively, which it catches by making shallow dives, or skimming the surface of the water (Courtney and Blokpoel 1979).
Conservation / Management
There are six primary breeding areas in Minnesota (Pine/Curry Island, NW Angle, Mille Lacs Lake, Duluth/Lake Superior, Leech Lake, and Cotton Lake), which should be censused annually. Intensive management efforts will likely be necessary to maintain or increase population levels at these sites. Efforts should focus on providing open beaches for nesting that are free from human disturbance, gull competition, and avian and mammalian predation (Reed et al. 1991; Penning and Cuthbert 1993). Creation of new nesting sites, including artificial nesting rafts as described by Dunlop et al. (1991), may be necessary to offset competition for nest sites with ring-billed gulls.
Conservation Efforts in Minnesota
Between 1980 and 1995, a concerted effort was made to monitor colony size and reproductive success of the common tern in Minnesota by state, federal, and tribal agencies. Intensive management efforts, including enhancing nesting habitat, controlling predation, and reducing competition from gulls, have occurred at all known colonies. During this time, colony size and reproductive success varied markedly between years. Researchers have learned that brightly colored nylon string works well for scaring ring-billed gulls away from areas where common terns are nesting without affecting the terns (Maxson et al. 1996), and that providing common tern chicks with shelters decreases predation by gulls (Burness and Morris 1992). These efforts are very labor intensive and have been most successful when done in collaboration with other agencies, such as at Interstate Island in the Duluth Harbor, which is managed cooperatively with the Wisconsin DNR. Cost effective strategies to maintain suitable nesting habitat for this species and to reduce predation are likely necessary to ensure the common tern's survival in Minnesota.