Forster's tern
Sterna forsteri
Identification Tips:
- Length: 14 inches Wingspan: 30 inches
- Sexes similar
- Dives into water for prey
- Medium-sized tern with slender, pointed bill
- Long, deeply forked tail
- Smoothly rounded head without crest
- Pale underwing with broad, blurry, dark trailing edge
- Gray upperwing with pale primaries (and variably dark outermost primaries,
dependent on plumage wear)
- Little translucence in flight feathers when seen from below
- May take three years to reach full adult plumage
Adult alternate:
- Orange legs
- Orange bill with black tip
- Black cap
- White face, foreneck, breast and belly
- Pale gray rump and tail, with dark inner edges and white outer edges to
longest tail feathers
- Tail extends just beyond tip of primaries at rest
- Plumage held in spring to early summer
- Molt to basic plumage starts in July and is nearly comlete by early fall
Adult basic:
- Orange legs
- Black bill
- White head, face, foreneck, breast, and belly
- Black patch surrounding eye
- Freshly molted primaries typically very pale
Juvenile:
- Pale orange legs
- Black bill
- White forehead, foreneck, breast, and belly
- Black eye patch
- Pale brown back
- No carpal bar
- Gray wings with brown tips to upperwing coverts
Immature:
- Black bill and legs
- First-year birds generally have dark secondaries and outer primaries, and
a slightly more extensive black mask
- Second-year birds are generally very similar to adults in alternate
plumage but often have darker primaries and secondaries and white tips to
feathers in black cap
Similar species:
When identifying terns, it is safest to rely on a combination of field
marks. In most plumages the Forster's Tern has two field marks that separate
it from the similar Common, Arctic, and Roseate terns: a black eye patch and
white primaries that contrast with the gray upperparts. In alternate plumage,
Common and Arctic terns have gray not white underparts while the Roseate Tern
often has a darker bill. Common and Arctic terns have dark outer edges of
the tail and white inner edges; just the opposite of the Forster's. The undersurface
of the primaries is a useful feature for separating the Forster's Tern from
Arctic and Roseate terns. The Forster's Tern has a broad, blurry trailing
edge to the primaries where it is thin and crisp in the Arctic and very
restricted in the Roseate. On the upper surface of the primaries, the Common
Tern has a dark wedge that is lacking in the Forster's Tern. The larger
Sterna terns (Royal, Elegant, Caspian) have entirely orange or red bills, much
larger bodies, broader wings, shorter tails and ragged crests at rear of head.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.