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LEAST TERN Sternula antillarum 21–23cm. Tiny tern of inshore marine
waters, beaches, river mouths. Feeds mainly over inshore waters; often rests on
beaches with other terns, smaller gulls, shorebirds. Flies with hurried deep
wingbeats and plunge-dives steeply from moderate heights. Tiny size distinc
tive—barely larger than a Sanderling. SOUNDS: Varied, slightly sneezy and
squeaky calls, typically 2-syllable, chírit and kree-it. STATUS: Uncommon to
seasonally fairly common nonbr. migrant over inshore Pacific waters, especially
Gulf of Nicoya, mainly Sep–Apr; a few imms. may occur through summer.
Rare and irregular on Caribbean coast. (Breeds N America to Mexico, winters
w. Mexico and s. Caribbean to n. S America.)
COMMON TERN Sterna hirundo 29–32cm (+ 2.5cm adult tail streamers).
Coastal and offshore winter migrant, often resting at beaches, river mouths,
coastal lagoons, harbors; feeds mainly over inshore marine waters. Singles to
locally in flocks of 100s off Pacific coast, often mixed with other terns (especially
American Black Tern). On nonbr./imm. note partial black cap with white
forecrown, blackish leading edge to wing (shows at rest as dark shoulder bar).
Breeding adult has red bill with small dark tip, pale smoky wash to body. Post-
juv. plumages often have dark wedge on trailing edge of primaries (can be
indistinct in spring, and also shown by 2nd-year Forster’s). Wing molt occurs
fall–winter, in Costa Rica. SOUNDS: High sharp kiik, suggesting Long-billed Dowitcher. STATUS: Fairly
common to common Aug–May along and off Pacific coast, smaller numbers of imms. occur locally in
summer. Uncommon spring and fall on Caribbean coast, scarce in winter. (Breeds n. Eurasia and N
America, winters to S Hemisphere.)
ARCTIC TERN Sterna paradisaea 30–33cm (+ 3–5cm adult tail streamers). Pacific, offshore transient
migrant. Singles rarely found resting on beaches with other terns, when note very short legs of Arctic.
Stockier than Common Tern, with shorter neck, longer wings, and often stronger, more clipped wingbeats.
On adult, note uniform translucent primaries with narrow black trailing edge, smoky-gray body; bill
slightly shorter than Common (can have black tip on 2nd-summer and fall adult). Juv. has whitish
secondaries (dark on Common) contrasting with darker leading edge of wing, but lacks contrasting black
shoulder bar. Wing molt in S Hemisphere, not in Costa Rica. STATUS: Probably a regular migrant Aug–
Nov, Apr–May far off Pacific coast, rarely wandering to within range of day trips; scattered coastal records
mainly Oct–Nov. (Breeds n. Eurasia and N America, winters S Hemisphere.)
FORSTER’S TERN Sterna forsteri 32–36cm (+ 6–7.5cm adult tail streamers). Scarce winter migrant to
estuaries, coastal lagoons, harbors, beaches, wetlands; not usually offshore. Likely to be found as single
birds, often resting with other terns, gulls, shorebirds. Slightly larger and bigger-billed than Common
Tern; nonbr./imm. plumages have distinctive broad black mask on white head, orange-red legs; cf. Gull-
billed Tern. Breeding adult has silvery upperwings, whitish body, orange-red bill with extensive black tip,
long tail streamers; wing molt in fall, completes before winter. Cf. Common Tern. SOUNDS: Hard clipped
kik! STATUS: Rare and irregular nonbr. migrant to n. Pacific coast, mainly Nov–Feb in Gulf of Nicoya; very
rare on Caribbean coast. (Breeds N America, winters to Cen America.)
GULL-BILLED TERN Gelochelidon nilotica 33–36cm. Medium-size, rather
stocky tern of coastal lagoons, salt ponds, beaches, flooded fields, lakes; not over
open ocean. Feeds by swooping down to snatch prey (crabs and such) from
ground, not by diving into water. Associates readily with resting groups of other
terns, gulls, skimmers. Distinctive: note habits, thick black bill, relatively long
legs, short tail, very pale gray upperparts (no white rump), tapered and swept-
back wings. Cf. nonbr./imm. Forster’s Tern, Sandwich Tern. SOUNDS: Nasal
laughing and mellow barking calls, mainly in flight, ku-wek and ket-e-wek, etc.
STATUS: Uncommon and local nonbr. migrant on both coasts, mainly Sep–May,
a few imms. may occur in summer. (Worldwide.)
