Forster's Tern

About

Forster's Tern is a medium-sized tern found in North and Central America. It is known for its elegant flight, distinctive calls, and preference for coastal and inland wetlands. During the breeding season, adults display a striking black cap and orange-red bill and legs. It is a highly migratory species, with populations wintering along the coasts of the southern United States, Mexico, and Central America.

Size & lifespan

Length

33-36 cm (13-14 in)

Wingspan

76-82 cm (30-32 in)

Weight

130-190 g (4.6-6.7 oz)

Lifespan

Up to 12 years in the wild

Diet

Carnivore (primarily piscivorous)

Physical characteristics

Plumage

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.

Beak shape

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

Eye color

Dark brown

Leg color

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

Distinguishing features

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.

Juvenile appearance

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.

Seasonal changes

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.

Sexual dimorphism

Males and females are outwardly similar in plumage and size.

How to identify

1. 1

   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.

2. 2

   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.

3. 3

   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.

4. 4

   Tail Shape

   Note the long, deeply forked tail, especially prominent when the bird is in flight or hovering.

5. 5

   Leg Color

   Breeding adults have orange-red legs, while non-breeding adults and juveniles have blackish legs.

6. 6

   Vocalizations

   Listen for the harsh, grating 'karrr' or 'zraah' calls, which are distinctive.

Habitat & range

Biomes

Coastal wetlands, estuaries, salt marshes, freshwater marshes, lakes, rivers, and coastal beaches.

Geographic range

Breeds across North America, primarily in the interior and along the Atlantic and Gulf Coasts. Winters along the Pacific, Atlantic, and Gulf Coasts of the United States, Mexico, and Central America.

Elevation range

Typically found from sea level up to a few hundred meters, primarily in lowland wetland habitats.

Preferred environments

Shallow coastal bays, estuaries, salt marshes, and large freshwater marshes with emergent vegetation for nesting. During migration and winter, they favor sheltered coastal waters, lagoons, and tidal flats.

Behavior

Activity time

Diurnal, actively feeding during daylight hours.

Feeding habits

Primarily piscivorous, feeding on small fish, often caught by shallow plunge-diving from a hovering position. They also consume crustaceans and aquatic insects. They typically hunt in shallow water, often in groups.

Social behavior

Highly colonial breeders, nesting in dense colonies. Outside the breeding season, they are often found in flocks, sometimes mixed with other tern species, roosting on sandbars or mudflats.

Flight pattern

Graceful and buoyant flight with deep, deliberate wingbeats. They often hover over water before plunge-diving headfirst to catch prey. The deeply forked tail is noticeable in flight.

Migration pattern

Strongly migratory. Northern populations migrate south for the winter, while some southern populations may be resident or undertake shorter movements. They migrate along coastlines and major river systems. In the San Francisco Bay Area, they are primarily a wintering and migratory species, with some non-breeding individuals present year-round.

Territoriality

Highly territorial during the breeding season, defending nest sites within colonies. They will aggressively mob predators or intruders near their nests.

Breeding & reproduction

Breeding season

Typically May to July, varying by latitude.

Nesting habits

Nests in colonies, often on floating mats of vegetation, muskrat lodges, or small islands in salt or freshwater marshes. The nest is a shallow scrape lined with vegetation, sometimes built up into a small mound.

Clutch size

2-4 eggs, most commonly 3.

Egg description

Typically 2-4 eggs, buff to olive-brown, heavily spotted and blotched with dark brown, black, or purplish markings.

Incubation period

23-26 days, incubated by both parents.

Fledging period

22-28 days after hatching, young are able to fly.

Parental care

Both parents share incubation duties and care for the chicks, feeding them small fish until they fledge and for a period thereafter.

Where to find

Field tips

1. Scan Open Water. Look for them actively plunge-diving for fish over shallow water or hovering gracefully before a dive.
2. Check Roosting Flocks. Examine flocks of terns roosting on mudflats, sandbars, or jetties. Look for the distinctive head patterns (black cap in summer, eye-patch in winter) and bill coloration.
3. Listen for Calls. Their harsh calls are often the first indication of their presence, especially when they are disturbed or interacting.
4. Seasonal Timing (San Francisco Bay Area). Forster's Terns are present year-round in the San Francisco Bay Area, but their numbers swell significantly during fall and spring migration, and they are abundant as winter residents. Breeding occurs further inland or south, but non-breeding individuals may be seen locally during summer.
5. Binoculars and Scope. Good optics are essential for distinguishing field marks, especially the subtle differences from similar tern species.

Similar species

Quick facts

Primary Diet:

Small fish

Nesting Habitat:

Salt and freshwater marshes

Migration Status:

Highly migratory

Conservation Status:

Least Concern

Distinctive Feature (Breeding):

Orange-red bill with black tip

Distinctive Feature (Non-breeding):

Black eye-patch and white forehead

Fun facts

• Forster's Terns are named after Johann Reinhold Forster, a German naturalist who accompanied Captain Cook on his second voyage around the world.
• They are one of the few tern species that regularly nests in freshwater marshes, often utilizing muskrat lodges as nesting platforms.
• The 'window' effect on their outer primaries in flight is a unique and reliable identification feature, especially when seen from below.
• During courtship, males often present fish to females as part of their display.

Scientific classification

Kingdom

Animalia

Phylum

Chordata

Class

Aves

Order

Charadriiformes

Family

Laridae

Genus

Sterna

Species

Sterna forsteri

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