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JAEGERS (STERCORARIIDAE; 3+ SPECIES) Small family of rather gull-like oceanic

birds that feed mainly by pirating other birds, especially gulls, terns, shearwaters. Ages differ, sexes

similar with females averaging larger. Adult plumage attained in about 3 years. ID of imms. can be

very difficult, best to focus on structure and a few key plumage features, especially shape of any tail

projections, extent and pattern of white wing flashes. Known as skuas in Old World.

POMARINE JAEGER (SKUA)  Stercorarius pomarinus 44–51cm (+ 6–11cm

adult tail projections). Pacific and Caribbean, offshore. Largest jaeger, chases

mainly large shearwaters and gulls, less often terns. Direct flight rather heavy

and steady, with powerful wingbeats; chases rarely prolonged and aerobatic.

Sometimes scavenges at fishing boats; can be found with rafting flocks of

shearwaters. Note relatively big head and bill, broad wings, and broad, blunt-

tipped tail projections. All ages can be dark overall with reduced white wing

flashes; adult dark morph uncommon. 1st-year has heavily barred underwings,

2nd-year has reduced barring on underwing coverts, 3rd-year and older

typically have solidly dark underwing coverts. STATUS: Uncommon to fairly common nonbr. migrant to

Pacific waters, mainly Aug–Apr; usually well offshore. Probably uncommon Sep–Apr in Caribbean, but

few records. (Breeds n. Eurasia and N America, winters widely at sea.)

PARASITIC JAEGER (ARCTIC SKUA)  Stercorarius parasiticus 40–44cm (+ 6–​

10cm adult tail projections). Pacific and Caribbean, mainly offshore; unlike

other jaegers, regularly seen from shore. Chases mainly terns, smaller gulls.

Flight strong and direct, suggesting a falcon; chases often persistent and aero­

batic. Note relatively small head, slender bill, sharply pointed tail projections,

crescent of white primary shafts on upperwing. All ages can be dark overall with

reduced white wing flashes; adult dark morph fairly common. Juv. often

relatively rusty-toned, unlike colder-toned juv. Pomarine and Long-tailed.

Ageing as in Pomarine Jaeger. STATUS: Uncommon to fairly common nonbr.

migrant to Pacific waters, mainly Aug–Oct, Mar–Apr, small numbers in winter; scarce off Caribbean

coast. (Breeds n. Eurasia and N America, winters widely at sea.)

LONG-TAILED JAEGER (SKUA)  Stercorarius longicaudus 37–41cm (+ 14–

21cm adult tail projections). Pacific, offshore. Smallest, most lightly built jaeger;

chases mainly terns, small gulls, phalaropes, storm petrels. Flight strong but

graceful, with buoyant wingbeats, suggests a smaller tern. Note relatively short

neck, small bill, only 2–3 white primary shafts on upperwing, finely pointed tail

projections of older imm. and adult; juv. has relatively long but bluntly-tipped

tail points. Juv./1st-year can be dark overall, but adult only has light morph.

Ageing as in Pomarine Jaeger. STATUS:  Scarce nonbr. migrant, usually well

offshore, mainly Aug–Nov, Mar–May. (Breeds n. Eurasia and N America,

winters at sea mainly in S Hemisphere.)

GULLS (LARIDAE; 5+ SPECIES) Worldwide family of familiar web-footed birds

often found near water. Ages differ, sexes similar but males average larger, bigger-billed. Adult

appearance attained in 2 years for smaller species (hence, 2-year gulls), up to 4 years or longer in

large species (4-year gulls); 2nd- and 3rd-year plumages highly variable, intermediate between

1st-year and adult. Seasonal variation mainly in head pattern, bill color and pattern. Few species

regular in Costa Rica, but several species occur as vagrants or rare migrants (Appendix B). Varied

crowing and mewing calls mainly in interactions, mentioned only when obviously useful for ID.

SABINE’S GULL  Xema sabini 32–35cm. Pacific, offshore. Small, boldly marked

2-year gull rarely seen from land. Singles or small groups, sometimes in association

with feeding groups of terns, other gulls, shearwaters. Flight direct and buoyant,

rather tern-like. Distinctive and striking in flight with gleaming white triangles

on upperwings; also note forked tail. Adult attains dark slaty hood in late winter;

juv. molts over 1st winter into adult-like plumage with partial dark hood or

hindneck patch. STATUS: Uncommon nonbr. migrant offshore, mainly Aug–Nov,

Mar–May. (Breeds n. Eurasia and N America, winters mainly S Hemisphere.)