Montana Field Guide

Least Tern - Sterna antillarum

The smallest tern in North America, the Interior Least Tern averages 21 to 24 cm long and has a wingspan of 51 cm (Thompson et al. 1997). In breeding plumage the species is characterized by a black cap and stripe through the eye that contrast sharply with a white forehead (Thompson et al. 1997). The underparts of the bird are white, while the upperparts are gray. The outer primaries of their long, narrow wings are black. They have a short, slightly notched tail, and a slightly decurved and tapered yellow bill (unique from other tern species) with a small black tip. The sexes are virtually identical, although Whitman (1988) notes some subtle differences; the male bill is described as orange to bright yellow, while the female's is light, dull yellow, or straw-colored. The iris is dark brown (Thompson et al. 1997); the feet and legs of the male are bright orange and generally bright to pale yellow on the female (Whitman 1988).

Vocalization of the Interior Least Tern is described as a shrill, rapid, sharp "piDEEK-adik" or "keDEEK" as well as a weak, nasal sounding "whididi" and high, sharp "kweek" or "kwik" squeaks. The alarm call is a sharp, rising "zreek" (Sibley 2000).

It is unlikely for the Interior Least Tern to be confused with any other tern species in Montana. Its diminutive size, yellow bill, and white forehead are distinctive. Another tern species found in the state, the Forster's Tern, also has a black cap, but it lacks the white forehead. Also, the Forster's Tern is larger than the Interior Least Tern, has a large orange, not yellow, bill and lacks black primaries in breeding plumage (Sibley 2000).

Montana Range

Little is known about the migratory patterns of the Interior Least Tern in Montana. Most of the observations in the state have been recorded for breeding pairs, with few reported sightings of transient individuals. Spring arrival of the species occurs in mid-May, with departure in the fall generally occurring by mid-August (Montana Bird Distribution 2003). The extreme early migration date in Montana is May 24, 1994 at Fort Peck (Skaar et al. 1985). Terns nesting in the northern U.S., including Montana, may migrate the farthest of all terns, possibly wintering in South America (Thompson et. al 1997).

Interior Least Terns nest on unvegetated sand-pebble beaches and islands of large reservoirs and rivers in northeastern and southeastern Montana, specifically the Yellowstone and Missouri river systems (Christopherson 1992). These wide, open river channels, and lake and pothole shorelines provide the preferred characteristics for nesting terns. Sites with gravel substrate provide the most suitable sites for nesting (MPPRC 1994). One of the most limiting factors to nesting site selection is vegetational encroachment; terns avoid areas where relatively thick vegetation provides cover for potential predators. Fine-textured soils are easier to treat mechanically than rocky or gravelly soils when vegetation is determined as a limiting factor in an area's ability to provide suitable nesting habitat, but fine soils are not typically a preferred nesting substrate (MPPRC 1994).

In Montana, as in other areas, another and more important limiting factor in nest site selection is the location of nesting sites in relation to surrounding water levels. Nests are often inundated because water levels are kept unnaturally high throughout the breeding season (and high winds can cause nests to be flooded) or nesting sites are not available (either because of encroaching vegetation or because water levels are so high that beaches are under water during the early part of, and possibly throughout, the nesting season) (MPPRC 1994).

No specific information regarding food habits in Montana is known for this species. Generally the Interior Least Tern consumes small fishes (generally less than 9 cm long), but sometimes eats crustaceans or insects. Prey is obtained by diving from the air into shallow water usually less than 4 m deep (Moseley 1976). Interior populations depend almost entirely on cyprinids. Feeding in newly plowed fields has been observed in Texas; apparently beetle larvae were being captured (McDaniel and McDaniel, 1963, Auk 80:544).

When breeding, the Interior Least Tern usually forages within a few hundred meters of the colony, but occasionally up to 3 to 12 kilometers away (summarized by Thompson et al. 1997). Coastal breeding populations may forage in marine, estuarine, or nearby freshwater habitats.

Interior Least Terns are a recent breeder in the state. The first confirmed nesting occured in 1987 (Montana Piping Plover Recovery Committee 1989, 1994). In Montana, nesting colonies are particularly vulnerable to inundation in the Missouri River system.

Research suggests that the Interior Least Tern travels in large flocks during migration; while in the non-breeding season the species is usually found singly or in small loose groups. Foraging may occur singly, in pairs, or in small flocks (Erwin 1978).

In California, the species usually nests in the same area in successive years; and tends to return to the natal site to nest (Atwood and Massey 1988). On Long Island, New York this species tends to nest in the same area in successive years if physical conditions are conducive to nesting (MacLean et al. 1991).

Interior Least Terns are known to nest along the shorelines of Fort Peck Reservoir, the Missouri River below Fort Peck Dam, and along the Yellowstone River, downriver from Miles City (Hanebury, Pers. Comm. 2003). Interior Least Terns have been observed nesting in association with Common Terns at Fort Peck Reservoir and are often closely associated with Piping Plovers in the limited areas of the state in which they breed (MPPRC 1994).

The highest nesting population of Interior Least Terns in the state, in most years, is found on the Missouri River below Fort Peck Dam and may number over 100 individuals (Casey 2000); this is particularly true when downriver conditions are not ideal and the terns move upriver in search of better nesting habitat (Pavelka, Pers. Comm. 2003). Nests may be lost to flooding when water levels rise during the breeding season as occurred in 1994 and 1995, when a labor-intensive captive hatching and release program saved eggs from nests imminently threatened with inundation by high water levels (Pavelka, Pers. Comm. 2003).

Over the past several years the total number of Interior Least Terns on the Missouri River has remained fairly constant, though the total number of initiated and successful nests is highly dependent upon river conditions. In 2003, 38 Interior Least Terns were observed on the Missouri River (this includes the portion of the river to the headwaters of Lake Sacajawea in North Dakota), with 19 nests initiated, 10 nests hatched, and 12 young fledged (Pavelka, Pers. Comm. 2003). Since 1998, the greatest number of birds observed on this stretch of the river (1999) totaled 40 birds (12 nests; 7 nests hatched; 17 chicks fledged). The highest number of nests initiated over this time period occurred in 2002, with 34 birds with 23 nests (10 nests hatched and 10 fledged) (Pavelka, Pers. Comm. 2003). The Yellowstone River nesting population is generally limited to less than thirty birds (MPPRC 1994).

As identified in the Recovery Plan for the Interior Least Tern, delisting can be considered when four censuses confirm that the interior population has reached 7,000 and remains stable for at least 10 years. The goal for the Missouri River system is 2,100 birds (census numbers in 2003 revealed 735 birds for the Missouri River in total; Pavelka 2003), with 50 individuals as the minimum targeted for Montana's population. This goal was exceeded from 1989 through 1992, dropped to only 39 individuals in 1993, and again met goal in 1994, when a record 51 pairs produced 81 young (MFWP 2003). For the past six breeding seasons (from 1998 to 2003), the goal of 50 Interior Least Terns for the Missouri River in Montana has not been met (Pavelka, Pers. Comm. 2003). Appropriate water management, that which includes natural seasonal flows, is identified as the major consideration for Interior Least Tern conservation in Montana, for the greatest threat to breeding pairs, in some years, is the loss of existing nesting sites from inundation by high water at unusual times of the breeding season (MPPRC 1994). Rising water levels late in the nesting season can also decrease overall island size, and may result in assisting local avian predators to locate tern nests (containing eggs or nestlings) more easily (Erickson and Prellwitz 1999). These conditions reinforce the need to manage reservoirs and dammed rivers in a manner that mimics more natural seasonal fluctuations for the protection of Interior Least Tern populations. Other management activities beneficial to the species include: instituting grazing management practices more appropriate to the conservation of the Interior Least Tern; controlling access to key nesting locations; moving nests upslope from areas where flooding of nests is imminent; relocating eggs to nests of other terns for foster incubation; signing of beaches to indicate nesting by Interior Least Terns (though in areas where there is hostility toward the species, or toward listed species in general, this is not recommended); beach enhancement (grading or burning to remove unwanted encroaching vegetation); raising island elevation to make room to move nests in years with rising water during the nesting season (MPPRC 1994); and timing spring flow releases from Fort Peck Dam to more closely mimic the natural seasonal flows of the river (MFWP 2003). Other management activities to enhance habitat or affect better protection for this species includes reducing human, dog, and vehicular disturbance during nesting (MFWP 2003).