Merlin

Forster's Tern Life History

Habitat

Habitat Marshes

Forster’s Terns inhabit freshwater, brackish, and saltwater marshes during the breeding season, when they nest in colonies around marshy edges and small islands free from predators. Some colonies nest directly on floating vegetation, others in high parts of the marsh where there is “wrack” (decaying vegetation deposited by wind or tide), and others on weedy, sandy, shelly, or pebbly beach adjacent to marshes. Most colonies are in wetlands larger than 50 acres where there is plenty of open water for foraging, but locations change with varying water levels, storm damage, and disturbance by predators and humans. After the young fledge, many Forster’s Terns from inland populations gradually migrate toward ocean coasts. Migrants may be found in almost any wetland context, from the shorelines of the Great Lakes to major rivers to fairly small freshwater marshes. In the southern part of the wintering range (along the Gulf of Mexico coast), many Forster’s Terns remain in inland freshwater marshes all season. Others join coastal populations of Forster’s, foraging in estuaries, lagoons, bays, and over open ocean. They roost on beaches and mudflats when not foraging. They winter farther north than any other species of tern in North America.

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Food

Food Fish

Like most terns, Forster’s feeds primarily on small (1–4-inch) fish, which they hunt from the air and capture by plunge-diving from heights as low as a few feet to as high as 50 feet or more. When foraging, most fly along shorelines or just offshore, about 20–25 feet above the water. They normally make a shallow dive, but in some cases, they take prey nearly a foot below the water’s surface, submerging the entire body. In saltmarshes where they breed, Forster’s Terns sometimes wait to forage until mudflats are covered with a few inches of incoming tide, when small fish are most accessible. Forster’s Terns are unusual among terns in that they sometimes hunt from perches on pilings, bridges, or utility wires. The birds watch the river or the tidal flow until they spot prey coming toward them, then fly from the perch and dive. Usually, they swallow small prey quickly, but may soften larger fish in the bill before swallowing. Forster’s Tern prey include shiner perch, yellow perch, sunfish, pike, stickleback, anchovies, sardines, gobies, menhaden, and silverside minnows. They also eat some insects during the nesting season, which they pick off the water or sometimes capture in flight.

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Nesting

Nest Placement

Nest Floating

Nests are made on the ground, in marsh vegetation, weeds, and wrack or windrows of dead vegetation, sometimes on floating vegetation or muskrat lodges.

Nest Description

Nests of this species vary considerably, from a shallow scrape with no lining to a rough bowl made of marsh plants (bulrush, cattail, sedge) with variable dimensions, roughly 7 inches across, with interior cup 5 inches across and 1 inch deep.

Nesting Facts
Clutch Size:1-4 eggs
Number of Broods:1 brood
Egg Length:1.3-1.9 in (3.35-4.7 cm)
Egg Width:1.1-1.3 in (2.9-3.25 cm)
Incubation Period:23-28 days
Nestling Period:2-7 days
Egg Description:Olive to buff, marked with numerous small spots and blotches of dark brown, often concentrated around the larger end.
Condition at Hatching:Downy, eyes open, able to walk but stays in nest.
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Behavior

Behavior Aerial Dive (water)

Soon after arriving in breeding areas in spring, Forster’s Terns begin courtship displays. Male and female of a new pair make coordinated flights high in the air, using exaggeratedly slow wingbeats, circling and rising until the male begins to descend, wings raised above the horizontal and bill pointed downward. The female follows, just above the male, who sometimes carries a fish for this display. Often, another male with fish will join the pair. Another flight display involves the male hovering, with tail spread, low over the colony, bearing a fish. Males regularly offer fish to females at these times in “courtship feeding” displays. The pair may perform other ground displays, such as parading in tandem through the colony, bills raised. Males bring females fish when females are incubating and also share incubation duties. Both adults defend the tiny nest territory in the colony, warding off approach by neighbors with harsh calls, open-bill gestures, raised or lowered head, and spread wings. Trespassers receive a peck. Colonies can be as small as two nests or as large as several thousand. Both adults feed young until several weeks after the young learn to fly. Family groups probably migrate together. Forster’s Terns are also social during the nonbreeding season, foraging in small to large flocks and roosting together peaceably on beaches, mudflats, and islands.

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Conservation

Conservation Low Concern

According to the North American Breeding Bird Survey, Forster’s Tern populations were approximately stable between 1966 and 2015. However, because many colonies shift locations from year to year, these trend estimates are less certain than for other species. Partners in Flight estimates the global breeding population to be 98,000 and ranks the species a 13 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Sea-level rise and the loss and modification of wetland habitats are the two most critical conservation challenges in the case of this species.

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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. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.

Lutmerding, J. A., and A. S. Love (2017). Longevity records of North American birds. Version 2017.1. Patuxent Wildlife Research Center, Bird Banding Laboratory, Laurel, MD, USA.

McNicholl, Martin K., Peter E. Lowther and John A. Hall. (2001). Forster's Tern (Sterna forsteri), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

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

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