Black Terns nest in large freshwater wetlands, usually in dense marshes on the edges of shallow lakes of the open prairies or northern forests. They sometimes nest in rice fields or on river islands. Black Terns normally select marshes that are 50 acres or larger for nesting. Migrants turn up in many sorts of wetland habitats: sewage lagoons, river edges, lakes, marshes, lagoons, beaches, and over open ocean waters, even far out to sea. During the nonbreeding season, most Black Terns forage in tropical ocean waters with plenty of small fish, but they also frequent coastlines, lagoons, saltpans, estuaries, marshes, shrimp farms, and flooded farm fields, usually not far from the ocean.Back to top
Black Terns eat mostly small fish and insects. They feed by coursing slowly above marsh vegetation and open water, watching for prey, which they capture by swooping, then taking the prey in the bill. Black Terns don’t dive deeply into the water to capture prey the way other tern species do. They also capture flying insects, chasing after them in swift and erratic pursuit, much like a swallow or nighthawk. On occasion, they feed over fields on insects stirred up by tractors. During the breeding season, Black Terns eat grasshoppers, crickets, locusts, flies, ants, damselflies, dragonflies, mayflies, caddisflies, beetles, moths, spiders, crayfish, fish, tree frogs, and small lizards. During the nonbreeding season, they continue to eat insects and other small creatures onshore, but in offshore waters they prey almost entirely on small fish such as anchovies, sardines, and silverside minnows.Back to top
Male and female select the nest site together. This is usually well away from shore, in an area of shallow, still water sheltered from wind and waves, where the water’s surface is about half-covered in cattails, bulrushes, or other emergent vegetation. They often use areas with dead, floating vegetation on which to place the nest. Some nests are set on muskrat feeding platforms or lodges.
Both male and female arrange dead vegetation from the nest area into a shallow mound with a central bowl on top of floating vegetation. Nests average about 6.9 inches across and 1.6 inches tall, with interior cup about 3.5 inches across.
|Number of Broods:||1 brood|
|Egg Length:||1.2-1.6 in (3.09-3.95 cm)|
|Egg Width:||0.9-1.0 in (2.19-2.61 cm)|
|Incubation Period:||19-21 days|
|Nestling Period:||18-24 days|
Light buff to dark olive, speckled with gray to dark brown.
|Condition at Hatching:|
Covered in cinnamon to gray down with a pale belly and dark throat. Eyes open, able to lift head and vocalize soon after hatching.
Black Terns are among the most social of terns. When arriving on the breeding grounds, they perform group courtship flights high (up to 600 feet) in the air, ascending quickly with rapid, exaggerated wingbeats, then chasing each other in shallow swoops and dives with wings crooked, calling excitedly the whole time. Usually, these displays involve just a few to a dozen birds, but up to 300 birds may display together in courtship flights, which may last as long as 20 minutes. In some cases, Black Terns arrive on the breeding grounds already paired. Once they are paired, Black Terns do not display high in the air as often, but they begin courtship feeding. In this display, males fly slowly with a fish or insect in the bill, the female following. When the female perches, the male places the prey in her bill. On the ground, males court females by raising the neck and bill and dropping the wings, then dropping the head and raising the tail. The female sometimes mirrors these postures in return. Once the female lays eggs, most courtship ceases. After eggs hatch, some pairs begin their displays again. Black Terns appear to be monogamous in their mating system. Both males and females chase other Black Terns away from the nest vicinity, but conflicts are less common than in tern species that nest in dense colonies. Black Terns nest in loose groups, usually a few dozen pairs spaced about 30 feet apart, but some nests can be as close as 3 feet apart. Juveniles migrate at night with their parents toward wintering areas and remain in those areas during their first year of life, rather than returning northward in their first spring. During the nonbreeding season, Black Terns are highly social, occurring in flocks of a few to many thousands. They often roost along shorelines with other seabirds, especially other terns. They readily rest on pieces of flotsam and jetsam (and sometimes on the backs of resting sea turtles) at sea, but they rarely sit on the water’s surface.Back to top
According to the North American Breeding Bird Survey, Black Tern populations declined by an estimated 1.4% per year between 1966 and 2015, resulting in a cumulative decline of 51% over that period. Partners in Flight estimates a total breeding population of 850,000 (including the Eurasian range) and rates the species a 13 out of 20 on the Continental Concern Score, placing it on the Yellow Watch List for species with declining populations. The North American Waterbird Conservation Plan estimates a population of 100,000–500,000 breeding birds in North America. This species is difficult to monitor, as changing water levels often force nesting Black Terns to relocate from year to year or even during a single nesting season. The causes of population declines are not well understood but include drainage or conversion of wetlands and reduction of insect populations by agricultural pesticide use or other contributors to poor water quality. In Central American wintering areas, Black Terns may have been affected by sharp declines in prey species, especially small fish. Key to the conservation of this species is protection and restoration of freshwater wetlands from the Great Lakes region through the prairie provinces of Canada, where declines have been severe since the 1960s. Recommendations for wetland management include keeping water levels stable and maintaining a mosaic of emergent vegetation and open areas of water. In some places, Black Terns have nested successfully on artificial nest platforms, and have benefited from predator exclosures. Climate change forecasts suggest warmer temperatures throughout the breeding areas, with stronger storms and periods of drought, all of which could harm nesting habitat and nesting success in Black Terns.Back to top
Heath, Shane R., Erica H. Dunn and David J. Agro. (2009). Black Tern (Chlidonias niger), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
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Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.