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Comparing terns

Terns are quick and pale and all look alike at first glance. Yet a few features usually separate them: the colour of the bill (and whether it has a black or yellow tip), the legs, the head, the tail and where you see them. Tap what you see below; the list then shows only the species that fit everything.

• Within one row you can pick several values (red or orange legs); across rows everything must match.
• Everything refers to adult birds. The head changes with the season, so breeding and winter plumage are listed separately. Young terns are much harder; see the species pages for them.
• Bill and leg colour vary with season and light, and at a distance a bill often looks darker. If in doubt, pick two values.

Species	Length	Legs	Bill	Head, summer	Head, winter	Tail	Mantle	Underparts	Where	What to look for
Large terns
Caspian TernHydroprogne caspia	47–54 cm	Black/dark brown	Red/orange-red	Black cap	Streaked cap	Short	Pale grey	White/pale grey	Coast and inland	The largest tern, with a thick, bright red bill with a dark tip and dark undersides to the primaries; the cap stays streaked in winter. Royal Tern has a slimmer orange bill.
Royal TernThalasseus maximus	45–50 cm	Black/dark brown	Yellow to orange	Cap with crest	White forehead	Forked	Pale grey	White/pale grey	Coast and sea	Large crested tern with a thick, straight orange bill and pale underwings; outside the breeding season a broad white forehead, with the black behind the eye. Caspian Tern has a heavier red bill.
Sooty TernOnychoprion fuscatus	36–45 cm	Black/dark brown	Black	Cap, white forehead	White forehead	Long streamers	Black	White/pale grey	Coast and sea	Sooty black above and white below, with no pale collar; the white forehead patch stops above the eye. On Bridled and Gray-backed Terns the white continues as an eyebrow behind the eye.
Elegant TernThalasseus elegans	39–42 cm	Black/dark brown	Yellow to orange	Cap with crest	White forehead	Forked	Pale grey	White/pale grey	Coast and sea	Long, thin, slightly drooping orange bill and a long, loose crest; outside the breeding season the black still surrounds the eye. Royal Tern is bulkier, with a thicker, straight bill.
Medium terns
Sandwich TernThalasseus sandvicensis	34–45 cm	Black/dark brown	Black, yellow tip	Cap with crest	White forehead	Forked	Pale grey	White/pale grey	Coast and sea	Long, slender black bill with a yellow tip, black legs and a shaggy crest; bright white below and pale grey above. Gull-billed Tern has a shorter, thicker, all-black bill.
Roseate TernSterna dougallii	33–41 cm	Red	Black, red base	Black cap	White forehead	Long streamers	Pale grey	White/pale grey	Coast and sea	The palest tern, with white underparts, a long, mostly black bill with a red base and white tail streamers far beyond the wingtips; only the outer primaries are dark.
Gray-backed TernOnychoprion lunatus	35–38 cm	Black/dark brown	Black	Cap, white forehead	White forehead	Long streamers	Medium grey	White/pale grey	Coast and sea	Grey back without a pale collar, clean white underparts and a narrow white eyebrow reaching past the eye; Sooty Tern is black above, Bridled Tern brownish-grey with a pale collar.
Gull-billed TernGelochelidon nilotica	33–38 cm	Black/dark brown	Black	Black cap	Eye patch	Short	Pale grey	White/pale grey	Coast and inland	Short, thick, all-black bill without a yellow tip, long black legs and broad pale wings; hunts low over marsh and fields for insects and crabs rather than diving.
Aleutian TernOnychoprion aleuticus	32–39 cm	Black/dark brown	Black	Cap, white forehead	White forehead	Forked	Medium grey	Grey	Coast and sea	Grey breast and belly contrasting with a white rump and tail, a sharply defined white forehead in the black cap and a bill that is always black; in flight, a dark bar along the trailing edge of the secondaries.
Forster's TernSterna forsteri	33–36 cm	Orange	Red/orange-red	Black cap	Eye mask	Long streamers	Pale grey	White/pale grey	Coast and inland	Orange bill with a black tip and orange legs; silvery-white primaries, paler than the rest of the wing. In winter, a separate black eye mask on a white head with a white nape.
Common TernSterna hirundo	31–38 cm	Red	Red/orange-red	Black cap	White forehead	Forked	Pale grey	White/pale grey	Coast and inland	Orange-red bill with a black tip, medium-length red legs and a dark wedge in the outer primaries; at rest the tail reaches about to the wingtips, not beyond them as in Arctic Tern.
Arctic TernSterna paradisaea	28–39 cm	Red	Red/orange-red	Black cap	White forehead	Long streamers	Medium grey	White/pale grey	Coast and sea	Blood-red bill without a black tip, very short red legs, greyish underparts and tail streamers beyond the wingtips; translucent primaries from below with a narrow black trailing edge, unlike Common Tern.
Bridled TernOnychoprion anaethetus	30–32 cm	Black/dark brown	Black	Cap, white forehead	White forehead	Long streamers	Dark grey	White/pale grey	Coast and sea	Brownish-grey back, paler than the black cap, with a pale collar and a narrow white eyebrow reaching behind the eye; black bill and legs and a long, deeply forked tail.
Small terns
Black TernChlidonias niger	23–26 cm	Black/dark brown	Black	Head+body black	Eye patch	Short	Dark grey	Black	Marsh, fresh water	Black head and belly with slate-grey wings and tail, pale underwings and white undertail coverts; in winter a dark patch on the breast side and a grey, not white, rump.
White-winged TernChlidonias leucopterus vagrant	20–24 cm	Red	Dark red	Head+body black	Eye patch	Short	Black	Black	Marsh, fresh water	Black body and black underwing coverts with gleaming white forewing, rump and tail; dark-red bill and red legs. In winter it lacks the dark breast-side patch of Black Tern and shows a white rump.
Least TernSternula antillarum	21–23 cm	Orange	Yellow to orange	Cap, white forehead	White forehead	Short	Pale grey	White/pale grey	Coast and inland	The smallest tern, with a yellow bill with a black tip, yellow legs and a white forehead triangle in the black cap; flies with fast, stiff wingbeats and often hovers before diving.

The features come from the species accounts in this guide, supplemented from standard field guides. Lengths are those on the species pages and include the tail streamers; the groups are a convenience, not a fixed classification.