The Forster’s Tern (Sterna forsteri) is a medium-sized tern belonging to the Sternidae family. Often called the White-winged Black Tern, it breeds in marshes and wetlands across North America and migrates to coastal areas of Central and South America for the winter. Forster’s Terns are elegant flyers known for their aerial acrobatics and plunging dives for fish. They are distinctive with their black caps, white foreheads, bright orange-red legs and bills, and long, forked tails. Read on to learn more about the identification, behavior, habitat, diet, and conservation status of this beautiful sea bird.
Identification
Forster’s Terns are medium-sized sea birds measuring 33–36 cm in length with a 76–81 cm wingspan. Their plumage changes seasonally. In the breeding season, adults have glossy black crowns, white foreheads, and black napes. Their backs and upperwings are pale gray, and their underparts are white. The outer primaries of their wings are silvery-gray with black tips. Their forked tails are long and white with black outer webs. Their legs are bright orange-red, as are their thin, sharp bills. In non-breeding plumage, the black crown becomes speckled with white and the bill turns mostly black with a reddish-orange base.
Immature Forster’s Terns resemble winter adults but have dusky carpal bars on their wings. Young birds molt into adult plumage by their second year. Though similar looking, Forster’s Terns can be distinguished from other terns by their orange legs, all-black bills, and forked tails.
Behavior
Forster’s Terns are migratory birds that breed in North America and winter along the Pacific and Atlantic coasts of the Americas. They migrate in large flocks, sometimes joining other tern species. Their breeding season lasts from April to August. Forster’s Terns are colonial nesters, gathering in groups near coastal and inland wetlands. Breeding colonies can number several hundred pairs.
Males establish nesting territories and perform courtship rituals such as aerial displays, fish offerings, and nest preparations to attract a mate. Once paired, the male and female both help build a shallow nest scrape in the ground, lined with grasses and debris. Nests are typically built on islands for protection from predators.
Forster’s Terns lay 2-3 eggs per clutch. Both parents incubate the eggs for about 25 days. The chicks hatch downy and rely on their parents for food and protection. They fledge at 4-5 weeks old but remain dependent on their parents for an additional few weeks. In some cases, Forster’s Terns may raise two broods in a season.
Forster’s Terns feed mainly on small fish and sometimes insects. They forage by hovering 10-20 meters over the water and plunge-diving to catch prey. They also snatch food from the water’s surface while flying. Their excellent aerial agility aids in hunting.
These highly social terns gather in large flocks outside of breeding season. Groups may congregate in estuaries and harbors or roost on beaches and jetties. Their call is a loud, rolling creek.
Habitat
Forster’s Terns nest in a variety of coastal and inland wetland habitats across their breeding range, favoring areas with low, exposed islands for colony sites. Typical nesting locations include salt marshes, lagoons, ponds, lakes, reservoirs, and rivers. They prefer sites with shallow, open water that allow for plunge-diving.
During migration and winter, Forster’s Terns frequent coastlines, estuaries, bays, coastal lakes, and river mouths from Canada to Chile. They roost on sandy beaches and rocky shores. On their wintering grounds, they utilize both marine and freshwater habitats.
Diet
The diet of Forster’s Terns consists predominantly of small fish such as silversides, sticklebacks, anchovies, and minnows. They also eat some large invertebrates like shrimp and crabs. Their babies are fed whole fish carried in the parent’s bill.
Forster’s Terns hunt by plunge diving, typically from heights of 10-20 meters. They hit the water feet-first with their wings back. Sometimes they catch prey by dipping down to the surface while flying. Their diet varies based on location and prey availability.
| Prey Type | Examples |
|---|---|
| Small fish | Silversides, sticklebacks, anchovies, killifish, minnows |
| Large invertebrates | Shrimp, crabs |
| Insects | Dragonflies, damselflies |
Forster’s Terns are opportunistic foragers, capable of adapting to hunt whatever small prey is abundant in their habitat. Their excellent flying and diving skills aid them in catching fast-moving fish and invertebrates.
Conservation Status
Forster’s Tern populations declined in the early 20th century due to hunting for the millinery trade, but increased again after the Migratory Bird Treaty Act was passed in 1918. Their numbers dropped again from the 1960s to 1980s as wetland habitat was degraded and terrestrial predators increased.
However, Forster’s Tern numbers have rebounded in recent decades thanks to wetland restoration and conservation efforts. They are not globally threatened and remain widespread across North America. However, they are a Species of Special Concern in some U.S. states where breeding populations are small or isolated. Expanding wetland reserves and managing water levels at key nesting sites remain important for their continued success.
Key Facts
- Scientific Name: Sterna forsteri
- Length: 33–36 cm
- Wingspan: 76–81 cm
- Weight: 120-150 g
- Lifespan: 10 years
- Conservation Status: Least Concern
Appearance
- Black cap and nape
- White forehead
- Pale grey back and wings
- Deeply forked tail
- Orange-red legs and bill
Behavior
- Social, nesting in colonies
- Migratory, wintering along the Americas’ coasts
- Plunge dives for fish
- Graceful flyers
- Noisy calls
Habitat
- Nests in marshes, lakes, and ponds
- Migrates along coasts
- Roosts on beaches and jetties
Diet
- Small fish
- Invertebrates like shrimp
- Sometimes insects
Conclusion
In summary, the Forster’s Tern is an elegant, medium-sized tern best identified by its black cap, orange bill, and buoyant flight. It breeds across North America in wetland colonies before migrating down the Atlantic and Pacific coasts for winter. A successful hunter of small fish and invertebrates, the Forster’s Tern thrives when provided with suitable marshy habitat. With ample breeding sites and protected wintering areas, Forster’s Tern populations should remain healthy into the future.
