Montana Field Guide

Common Tern - Sterna hirundo

In breeding plumage, the Common Tern has an orange-red bill tipped in black and orange-red legs. The back, body, and wings are a silvery-gray with blackish primaries on the wingtips, evident during flight. The nape and cap are black and extend low enough on the head to contain the black eye before abruptly stopping at the white of the cheek and neck. The outer tail feathers on the forked tail are dusky. In non-breeding plumage the bill and legs lose their red coloration and are black. The cap no longer covers the forehead, leaving a white patch nearly to the top of the head (Sibley 2000).

The vocalization is described as numerous, varied, and of sharp, distinctive, and somewhat irritable timbre (Nisbet 2002). The most common call, the advertising call, is described as a down-slurring "keeyuur" or an up-slurred "keeuri" in addition to the "kip" or "tyik" call that is expressed during flock feeding, or during take-off and landing (Sibley 2000, Nisbet 2002).

Distinguishable from Forster's Tern (S. forsteri) by the Forster's whiter underparts, lighter primaries, lighter back coloration (and, hence, less difference in color between back and tail), and greater amount of black on a bill that is more orange than red-orange (Sibley 2000, Nisbet 2002).

Montana Range

Congregations of birds on the wintering grounds in April, and sometimes into May, suggest that spring migration occurs rapidly (Nisbet 2002). The earliest migration date for Common Tern in Montana is in April, but the most concentrated arrival of birds occurs in May. Breeding has been recorded in May, June, and July, with fall departure beginning in late August and continuing into September (Montana Bird Distribution 2003). The extreme migration dates for the Common Tern are April 23, 1993 at Freezout Lake and October 3, 1960 in Madison County (Reichel 1996). Normal migration periods in Bozeman are May 9 to 25 and September 5 to 25, with peaks on May 9 and September 15. The normal arrival date in Fort Peck is April 30, and in Billings, May 9 (Skaar et al. 1985).

Nesting in Montana generally occurs on sparsely vegetated islands in large bodies of water, such as Medicine Lake and Bowdoin National Wildlife Refuge. Nest substrate at these locations includes sparsely sandy, pebbly, or stony substrate, surrounded by matted or sparsely scattered vegetation (Casey 2000, Nisbet 2002). A study in the Lewistown District of the Bureau of Land Management documented that the Common Tern selected sites larger than 30 acres, with emergent vegetation covering more than 25% of the shoreline on all but one of the eight sites studied (Feigley 1997). All nesting occurred on islands (Feigley 1997).

Food habits of Common Terns have not been closely studied in Montana. In other breeding locations in their range, studies have shown the diet consists mainly of small fishes (sometimes also crustaceans and insects) obtained at the surface of the water by diving from the air. Common Terns are susceptible (especially females just prior to laying) to poisoning from dinoflagellate toxin accumulated in fishes (Nisbet 1983). A pair may defend feeding territory away from the nest, especially prior to incubation (Ehrlich et al. 1992).

No ecological data is available for this species in the state. However, in Massachusetts, loss of eggs and chicks was attributed to nocturnal desertion of nests by adults in response to predation by the Great Horned Owl (Nisbet and Welton 1984). The presence of mink can reduce reproductive success (Burness and Morris 1993). Mammalian predation generally eliminates nesting colonies, limiting continuous successful nesting to islands (Nisbet 2002).

During the nonbreeding season, Common Terns may be found singly or in small, loose groups, and sometimes in large flocks in migration (Stiles and Skutch 1989). Staging flocks of up to 10,000 individuals have been reported in the Great Lakes region (Nisbet 2002).

The majority of breeding activity in Montana occurs in the northern portion of the plains on islands within large lakes or reservoirs. Based upon known distribution and recorded observations, up to 50 breeding occurrences for this species are expected in the state (Reichel 1996). The areas with the highest recorded numbers of nesting Common Terns are Bowdoin and Medicine Lake national wildlife refuges. Limited numbers of nesting terns have been reported at Nelson Reservoir, Benton Lake National Wildlife Refuge, and Freezout Lake (Montana Bird Distribution 2003). Behavior suggesting breeding has been reported at other bodies of water within the prairie portion of the state including Lower Wild Horse, Mud Lake, Ward Reservoir, Two Forks Reservoir, Lake Elwell, Tiber Reservoir, and Halfway Lake in the Sands Waterfowl Production Area. Most colonies are under 50, with sizes ranging from 2 to 236 (Reichel 1997). Although rare, breeding has been recorded west of the Continental Divide; one report exists for the Kalispell area and another near Polson (Montana Bird Distribution 2003).

In general, the Common Tern is a colonial nesting species and may be found in colonies of tens or hundreds of pairs, though they may range from a few (rarely singly) to several thousand (Nisbet 2002). Their subelliptical eggs are smooth, non-glossy, cream, buff, or medium-brown with fine marks, blotches, specks or irregular lines of brown, black, or gray and 42x30 mm (Baicich and Harrison 1997, Nisbet 2002). Egg-laying usually takes place May to July, and clutch size is 2 to 3. Incubation, performed mainly by the female, lasts 21 to 27 days. The nest may be a simple scrape in the soil or sand and may be lined with grass, pebbles, or small sticks. Both sexes tend young, which may leave the nest after 3 days, returning only to brood, and first fly at about 4 weeks. The species may produce two clutches in one season, but the second brood rarely fledges. In New York, the breeding season was timed to overlap with a seasonal increase in food abundance, but food availability began to decline before the period of peak demand for food by chicks (Safina and Burger 1988). In a two-year study, fish abundance affected reproductive performance (Safina et al. 1988).

Management recommendations in Montana include providing adequate water levels to protect nesting islands from mammalian predators, managing water to better mimic seasonal fluctuations to prevent flooding of nesting sites, and minimizing human disturbance at nesting colonies during the nesting season (Casey 2000). None of these recommendations are currently in place specifically for the protection of Common Tern, although more natural water regimes are considered in the management plans for several dammed rivers in the state.