New York Natural Heritage Program
Least Tern
Sternula antillarum Lesson, 1847
Birds

General Description [-]
The Least Tern is the smallest North American tern weighing only one ounce and measuring about 9 inches in length. It is slate grey above and white below with a black cap and black extended eyeline.

Identifying Characteristics [-]
The Least Tern is the smallest North American tern weighing only one ounce and 9 inches in length (21-24 cm). The sexes appear similar; breeding adults are slate gray above and white underneath. They have a black cap and nape, white forehead, black line running from the crown through the eye to the base of the bill, orange-yellow bill often with a dark tip, white or grayish underparts, short deeply forked tail, and yellow-orange legs and feet. They have a black wedge on the outer primaries is conspicuous in flight (National Geographic Society 1983). Adults in winter plumage have a dingy cap, dark nape, a black line through the eye, a dark bill, and yellowish feet and legs (National Geographic Society 1983, Peterson 1990). Juveniles are pinkish-buff above, with brownish U-shaped marks on the back, a dusky crown, and a dark bar on the front part of the folded wing. First-summer birds resemble adults but retain the dark bar on the wing and have a dark bill and dark feet and legs, dusky primaries, a dark nape, and a black line through the eye (National Geographic Society 1983, Forbush 1927, Farand 1983). Vocalizations include a male contact call "ki-dik" given when bringing fish to mate or young (Olsen and Larsson 1995), a shrill "zreep" and harsh "kip, kip, kip" alarm calls, a recognition call between pairs or adult and young when returning to the colony that is described as "k'ee-you-hud-dut" in New York (Wolk 1974), as well as a few other calls given during brooding (see Thompson et al. 1997 for further description). The eggs are pale or olive buff with dark purplish-brown or blue-gray speckles and streaks. Nests consist of shallow scrapes in the sand, soil, or pebbles and may be lined with shell fragments, small pebbles, or bits of wood or grass once incubation has begun (Wolk 1974).

Characters Most Useful for Identification [-]
The Least Tern appears similar to other tern species but is much smaller and unlike many others, the black cap stops above the eye and instead a black stripe extends across the eye to the bill. The Least Tern is also unique in that the bill and legs are an orangish- yellow instead of the reddish-orange of other similar tern species.

Best Life Stage for Proper Identification [-]
Adults in the breeding stage are easiest to identify although juveniles and nonbreeding adults may also be distinguised from other species.

Behavior [-]
Least Terns nest in colonies that occasionally are large but tend to be less than 25 pairs (Thompson et al 1997). They may forage singly or in small groups (Thompson et al. 1997). They are capable and acrobatic fliers with a strong direct flight. When nesting areas are threatened by a predator or intruder they respond with alarm calls and sometimes aggressive aerial dives or mobbing (Thompson et al. 1997) sometimes deficating on the intruder (Gibbons pers. comm.). Least Terns forage throughout the day. They forage by flying or hovering over waterbodies and dive, only partialy submerging, to catch fish in open mandibles (Thompson et al. 1997). They immediately resume flight and consume prey on the wing. Pairs are monogamous for the season and about half keep the same mates between seasons (Kress and Hall 2004). Both males and females tend nests and rear chicks with females spending more time at incubating and brooding chicks (Keane 1987). Chicks are altricial at hatching, meaning they have limitied mobility and thermoregulating capabilities. They are, however, covered in down and have open eyes at hatching (Thompson et al. 1997).

Diet [-]
The diet of Least Terns consists primarily of small fish and occasional crusteaceans, although shrimp, insects, mollusks, and annelids may also be consumed (Atwood and Kelly 1984, Carreker 1985, Whitman 1988, Wilson 1991).
Least Tern Images
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The Best Time to See
Least Terns may best be observed from May through August during the breeding season. They are rarely seen before May or after early September on Long Island.
J F M A M J J A S O N D
Present Breeding
The time of year you would expect to find Least Tern present (blue shading) and breeding (orange shading) in New York.
Similar Species
  • Common Tern (Sterna hirundo)
    The Common Tern appears similar to the Least Tern but has darker reddish-orange legs and bill, instead of yellowish-orange legs and bill like the Least Tern. The Common Tern's black cap extends down to cover the eye, while the Least Tern has a cap that stops at the forhead and black stripe extending across the eye to the bill.