Gull-billed Tern Life History

Habitat

Gull-billed Terns in North America breed almost exclusively along the coast, close to water. Historically this species showed a preference for nesting in saltmarshes, but most pairs along the Atlantic and Gulf coasts now breed on sandy beaches, sandy barrier islands, or (in cases where human disturbance at beaches is high) on dredged-material islands. The Salton Sea, a salt lake in the interior of California, hosts one of the few inland breeding colonies in the United States. Gull-billed Terns at this site nest on eroded levees and small islands in the lake or on human-made islands and small rafts in brackish impoundments adjacent to the shoreline.

Atlantic breeders in the United States use the same coastal habitats during migration as they do while nesting. On wintering grounds, Gull-billed Terns frequent mudflats and sandbars associated with coastal lagoons. They also occur inland when flooded fields provide feeding opportunities.

Food

Gull-billed Terns eat mostly insects, marine invertebrates, and small vertebrates. This includes grasshoppers, dragonflies, lizards, amphibians, fish, small mammals, and occasionally bird chicks. They eat less fish than other tern species, and unlike other terns, they very rarely plunge-dive into water. Instead, they pick small fish off the surface. Gull-billed Terns are agile flyers that are more insectivorous than most tern species, often catching insects like grasshoppers, dragonflies, and moths in flight. They also catch prey items like fiddler crabs by plucking the prey at the bottom of a steep stoop. While insects and other invertebrates are the major food sources for this tern in many areas, some populations rely on fish and reptiles as their primary prey. And when presented with the chance, Gull-billed Terns feed opportunistically on the unguarded chicks of other bird species, often Least Tern chicks.

Nesting

Nest Placement

Gull-billed Terns typically nest in small to medium colonies (5–50 nests), together with other tern species, and on many occasions, Black Skimmers. Nesting habitats include sandy areas of barrier beaches, shell banks in coastal lagoons, and on saltmarsh islands. Beach-nesting pairs often select a nest site near grass tufts, sticks, large shells, or other objects that stand out in a barren sand or shell setting.

Nest Description

Both members of a pair contribute to creating a shallow scrape in the sand. On beaches, birds then place shells and other items around the nest rim. In other breeding habitats, nesting pairs place twigs, grasses, or sedge stems around the edge of the nest scrape. The outer edge of a nest is approximately 7–12 inches in diameter, with an internal diameter of 5–6 inches. The nest rim is about 1–3 inches high, and the cup depth ranges from 1 to 1.5 inches.

Nesting Facts

Behavior

Gull-billed Terns are monogamous and pairs stay together for many years. When birds arrive at a breeding site, but before establishing a colony, they perform a series of aerial displays. Small groups of 6–8 terns take part in the first displays—the birds fly together in a straight line for 100 yards, reverse direction, and then fly 100 yards in the opposite direction, and so on. These initial group displays then lead to pairs performing their own flights and focusing more on their nests.

This species seems less tolerant of disturbance, and less site-faithful, than other medium-sized terns. Adults patrol against egrets and herons (potential nest predators) even at the edge of breeding colonies, and aggressively defend their nests against Great Egrets, Snowy Egrets, Caspian Terns, large gulls, Peregrine Falcons, and humans.

Gull-billed Tern chicks often move away from the nest after just a few days, earlier than most other tern chicks. And although adult Gull-billed Terns do not swim or even rest on the water, chicks as young as 4–5 days old are good swimmers, capable of swimming 20 yards or more when needed.

Unlike most other terns, Gull-billed Terns very rarely plunge-dive into the water to catch fish or other prey. Instead, they perform a distinctive swooping flight to pick prey off the surface of water or land.

Conservation

Gull-billed Terns are uncommon in their North American range and their colonial nesting habit makes numbers hard to estimate with standardized surveys. The North American Breeding Bird Survey estimates the population remained stable from 1966 to 2021, but notes that there is low precision with this estimate. Inventories of breeding colonies have been erratic, making estimates of population trends difficult. Partners in Flight estimated the global breeding population of Gull-billed Tern plus Australian Tern (split from Gull-billed Tern in 2023) at 190,000 individuals and rated the species a 13 out of 20 on their Continental Concern Score, indicating a species of low conservation concern. In 2002, the North American Waterbird Conservation Plan estimated between 6,000 and 8,000 breeding birds in North America.

Human disturbance from boating and recreation is one of the major sources of nesting failure for this species. Disturbances can cause adults to abandon their nests, and very young chicks can die after being driven away from the nest area too soon. The deterioration or destruction of habitat, in the form of wetland draining and agricultural intensification, is the main cause for population declines in Europe. Changing water levels also impact Gull-billed Terns, either flooding colonies or drying out wetlands and making it easier for predators to access nesting islands. Alterations to weather patterns and sea-level rise are increasingly common effects of climate change.

Credits

Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Molina, K. C., J. F. Parnell and R. M. Erwin. (2014). Gull-billed Tern (Gelochelidon nilotica), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

North American Bird Conservation Initiative. (2014). The State of the Birds 2014 Report. US Department of Interior, Washington, DC, USA.

Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, Jr. Ziolkowski, D. J. and W. A. Link. The North American Breeding Bird Survey, results and analysis 1966-2013 (Version 1.30.15). USGS Patuxent Wildlife Research Center (2014b). Available from http://www.mbr-pwrc.usgs.gov/bbs/.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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