What is a gull-billed tern?

The gull-billed tern (Gelochelidon nilotica) is a medium-sized coastal seabird found across the world’s warmer coastlines. It gets its name from its gull-like appearance and broad, thick bill, which is a distinguishing feature of the species. Let’s take a closer look at this fascinating bird.

Quick Facts

• Scientific Name: Gelochelidon nilotica
• Other Names: Marsh tern, gull-billed gull, bay tern
• Size: 35-41 cm long, 95-110 cm wingspan
• Weight: 140-290 g
• Coloring: Gray upperparts, white underparts, black cap on head in breeding season
• Diet: Small fish, aquatic insects and crustaceans
• Range: Temperate to tropical coastlines around the world

Identification

The gull-billed tern gets its name from the thick, heavy black bill that resembles a gull’s bill. The bill is short but strong, and slightly downcurved. In breeding plumage, adults have a distinctive black cap and gray upperparts contrasting with white underparts. The legs are short and black. In nonbreeding plumage, the black cap is lost and the upperparts become more pale gray. Young birds resemble nonbreeding adults, but have a dark carpal bar on the inner wing. The gull-billed tern’s wingspan is around 95-110 cm across when fully spread.

While the stout bill sets this tern apart from other terns, the gull-billed tern can still be confused with some gull species when seen at a distance. Careful observation of the bird’s structure, flight style, and bill shape helps distinguish it. The gull-billed tern has longer, more angular wings and a more streamlined profile than gulls. It flies with steady wingbeats and often hovers before diving to catch prey.

Distribution and Habitat

The gull-billed tern has a very wide distribution across the world’s temperate to tropical regions. Its breeding range stretches across southern Europe, Asia, Africa, Australia, and the Americas. Northern populations migrate down to warmer coasts for the winter. Their habitat is primarily along saltwater coasts, estuaries, lagoons and beaches, but they can also be found at inland lakes, marshes and swamps with open water for feeding.

Some key habitats and breeding sites include:

• Salt marshes of the Atlantic Coast of North America
• Breeding sites along the Mediterranean and Black Sea coasts
• Mangrove swamps in Africa and Asia
• Coastal lagoons in Baja California, Mexico
• Beaches and wetlands in Australia

The gull-billed tern’s widespread distribution is aided by its adaptability to both marine and inland water environments across a broad range of latitudes and climates.

Migration

Northern gull-billed tern populations migrate long distances to reach wintering grounds, while tropical populations are often year-round residents. In North America, they breed along the Atlantic Coast from Virginia to Florida and throughout the Gulf Coast. Northern nesters migrate down to the Caribbean, Gulf of Mexico, and Latin America for winter. Late summer is spent fattening up for the strenuous migration ahead.

Migration occurs via coastal routes or over land. Maximum one-way migration distances exceed 2,500 miles between breeding and wintering grounds. Migrating flocks may congregate in large numbers at stopover sites to rest and feed. Their long, pointed wings make the gull-billed tern well-suited for sustained, efficient flight over huge distances.

Breeding

Upon reaching breeding grounds in spring, gull-billed terns form monogamous breeding pairs. They nest in scattered colonies, often near other tern and seabird species. Nesting sites are on open beaches, dunes, or in marshes on the ground. The nest is a simple scrape in the sand or dirt, lined with shells, debris or vegetation.

The female lays a clutch of 2-4 eggs, which are incubated by both parents for around 3 weeks. To protect against heat, flooding and predation, parents may move eggs around frequently. Chicks hatch covered in down and are able to leave the nest within a couple days. They are fed and cared for by both parents. After 3-4 weeks, chicks fledge and start flying. Complete independence is reached at 6-7 weeks old.

Gull-billed terns exhibit site fidelity, returning to nest in the same general area year after year. Successful pairs may reuse the same nesting site if conditions permit. Nesting periods vary across the range based on local conditions and climate.

Diet and Feeding

The gull-billed tern feeds primarily on small fish, aquatic insects, and crustaceans. Favorite prey includes minnows, sticklebacks, mullets, tadpoles, and shrimp. The stout bill is adapted for holding slippery fish while the tern manipulates it into position for swallowing head first.

Terns use a combination of hunting methods to catch prey:

• Plunge diving – Dropping directly out of flight to seize prey at or near the surface
• Dipping – Flying low over water to pick up prey without fully submerging
• Hawking – Snatching insects and other prey while flying
• Wading – Walking along shallow wetlands to stab or pick up prey

Small fish make up the majority of their diet. Insects and other invertebrates become more important when feeding chicks. The wide, flat bill allows gull-billed terns to effectively grip and swallow 10-15 cm long fish. Their salt gland also enables them to drink saltwater and feed far from fresh water sources.

Behavior

Gull-billed terns spend much of their time loafing or roosting when not actively feeding. Migration and breeding are very energy-intensive, so rest is important. Roosting may occur on open beaches, mudflats, pilings and jetties. Heat regulation is aided by urohidrosis – excreting on the legs to provide evaporative cooling.

Breeding colonies exhibit synchronized activity patterns. Birds leave en masse on foraging trips and later return together. Group foraging may increase efficiency in locating shoals of fish. This social synchronization also aids in predator detection and deterrence.

Vocalizations include various harsh croaking or grunting calls used for communication between mates and colony members. High-pitched alarm calls alert the colony to disturbances or predators. Non-vocal displays like the gull-billed tern’s hunched, forward threat posture also communicate aggression toward intruders.

Population and Conservation

The gull-billed tern has an extremely large global population estimated between 130,000 to 350,000 individuals. Population trends appear stable, so the species is evaluated as Least Concern on the IUCN Red List. Local populations may face threats from human disturbance, habitat loss, and predation. Increased flooding due to climate change may also impact nesting habitat in coastal areas.

Some specific conservation concerns include:

• Predation by foxes, raccoons, and gulls on breeding colonies
• Disturbance by beachgoers, vehicles, and unleashed pets
• Habitat degradation due to shoreline development and pollution
• Mercury contamination in north Atlantic populations

Protection and management of breeding sites is crucial for conservation. Public education can help reduce detrimental human disturbance. Predator exclusion, habitat improvement, and management of water levels may also benefit local reproductive success.

Fun Facts

• The oldest known gull-billed tern was 17 years old when it was re-captured and re-released by researchers.
• Gull-billed tern parents may sneak a quick swim break while breeding, leaving their mate to watch the eggs!
• Rare variant plumages include all-white and gray-and-white gull-billed terns.
• The eggs of gull-billed terns have an olive to buff coloration with dark speckling.
• Young chicks will beg for food from their parents by pecking the adult’s bill.

Breeding season timing and clutch size vary across the gull-billed tern’s range. The table above shows examples from different global populations.

Significance to Humans

Gull-billed terns face some threats from human activity and coastal development, but they are still a relatively common species. Their use of beaches, marshes and estuaries brings them into frequent contact with people.

Positives of human interaction:

• Beach-goers may observe tern courtship, nesting, and chick-rearing behaviors at close range.
• Birdwatchers appreciate added diversity gull-billed terns bring to coastal bird communities.
• Fishing services and tourism benefit from healthy coastal ecosystems that support tern populations.

Negatives of human interaction:

• Beach nesting makes eggs and chicks vulnerable to trampling, vehicles, and unleashed pets.
• Disturbance by beachgoers may disrupt nesting and cause colony abandonment.
• Pollution and habitat loss from shoreline development degrades nesting habitat quality.

With thoughtful management, gull-billed tern populations can thrive alongside human recreation and industry. Measures like leash laws, beach closures, and nesting area enclosures can allow both species to share the coastline.

Where to See Gull-billed Terns

Some of the best places to view gull-billed terns in North America include:

• Edwin B. Forsythe National Wildlife Refuge (New Jersey) – contains an important northern breeding colony
• Cape Romain National Wildlife Refuge (South Carolina) – year-round resident population
• Hunting Island State Park (South Carolina) – hundreds nest annually on beach
• Venice Rookery (Florida) – largest winter roost in state
• Corpus Christi and Laguna Madre (Texas) – vital breeding and migration habitat
• Bolsa Chica Ecological Reserve (California) – endangered California population breeds here

Since they are limited to coastal regions in the Americas, locating breeding, migrating, or wintering birds means visiting beaches, estuaries and wetlands along the Atlantic, Gulf and Pacific coasts. A good pair of binoculars will help birders distinguish them from other white-and-gray seabirds.

Conclusion

With its stout bill and distinctive voice, the gull-billed tern stands out from other terns. Despite its wide range and large population, it faces threats from habitat loss and human disturbance. This highly mobile seabird links ecosystems from pole to pole as it roams the world’s coastlines. Learning more about species like the gull-billed tern can inspire increased appreciation and protection for the amazing migrations and lives of seabirds.