How to identify the Forster's Tern

To correctly identify the Forster's Tern (Sterna forsteri), check each of these features. Birds can be tricky: habit, habitat, and call often confirm a sighting that a single photo cannot.

Size at a glance

Length

33-36 cm (13-14 in)

Wingspan

76-82 cm (30-32 in)

Weight

130-190 g (4.6-6.7 oz)

Physical features to check

1. 1

   Plumage

   Overall feather coloration and pattern.

   Breeding adults have a white body, pale gray upperparts, and a solid black cap extending to the nape. The primaries are silvery-gray with dark tips. Non-breeding adults have a white forehead, a black patch through the eye and nape, and a white body. Juveniles resemble non-breeding adults but have a brownish wash on the back and wings, and a dusky carpal bar.

2. 2

   Beak shape

   Shape and color of the bill: a key field mark.

   Slender, pointed bill. Breeding adults have an orange-red bill with a black tip. Non-breeding adults and juveniles have a black bill.

3. 3

   Eye color

   Iris color and any distinctive eye markings.

   Dark brown

4. 4

   Leg color

   Leg and foot color.

   Breeding adults have orange-red legs and feet. Non-breeding adults and juveniles have blackish legs and feet.

5. 5

   Distinguishing features

   Standout features that separate this species.

   Long, deeply forked tail, especially prominent in flight. In breeding plumage, the orange-red bill with a black tip and orange-red legs are key. In non-breeding plumage, the black eye-patch and white forehead are distinctive. The outer primaries are silvery-white on the underside, creating a 'window' effect in flight.

6. 6

   Juvenile appearance

   How young birds differ from adults.

   Juveniles have a dusky carpal bar on the upperwing, a brownish wash on the back and wings, and a black bill and legs. The black eye-patch is less defined than in non-breeding adults.

7. 7

   Seasonal changes

   Plumage variation between breeding and non-breeding seasons.

   Dramatic changes between breeding and non-breeding plumage. Breeding plumage (spring/summer) features a solid black cap, orange-red bill with black tip, and orange-red legs. Non-breeding plumage (fall/winter) features a white forehead, black eye-patch extending to the nape, and a black bill and legs.

8. 8

   Sexual dimorphism

   How males and females differ.

   Males and females are outwardly similar in plumage and size.

Key field marks

• Bill Color and Pattern (Breeding). Look for an orange-red bill with a distinct black tip. This is a key field mark for breeding adults.
• Head Pattern (Non-breeding). Observe the white forehead contrasting with a prominent black patch through the eye and extending to the nape. This 'bandit mask' is diagnostic for non-breeding adults.
• Wing Pattern in Flight. In flight, look for the silvery-white outer primaries on the underside of the wing, creating a 'window' effect. This is a reliable distinction from Common and Arctic Terns.
• Tail Shape. Note the long, deeply forked tail, especially prominent when the bird is in flight or hovering.
• Leg Color. Breeding adults have orange-red legs, while non-breeding adults and juveniles have blackish legs.
• Vocalizations. Listen for the harsh, grating 'karrr' or 'zraah' calls, which are distinctive.

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