Least Tern Life History

Habitat

Least Terns nest in colonies on sandy, shelly beaches or islands on coastlines and rivers. They sometimes also nest in gravel pits, on dredge spoil, on flat gravel rooftops, or on dry mudflats. On rare occasions, parking lots, agricultural fields, and airports have hosted small colonies. In all of these settings, vegetation is sparse or absent. Generally, Least Terns return each year to past nest sites, but changes in nearby prey availability, predators, human activity, or substrate conditions can prompt them to move to other sites. For feeding, Least Terns use almost any aquatic environment, including oceans, bays, estuaries, rivers, streams, sloughs, dike fields, marshes, ponds, sand pits, and reservoirs. Little is known about Least Tern migration, but migrants are observed along ocean coastlines, river corridors, and occasionally far offshore. Wintering Least Terns in South America occupy ocean coastlines, bays, and estuaries, often near rivermouths.

Food

Least Terns feed almost entirely on small fish. They catch fish by diving, usually in fairly shallow water, often hovering briefly before diving. They also occasionally eat shrimp, tadpoles, flying insects, and ants. Prey fish include sandlance, herring, hake, anchovy, menhaden, silversides, killifish, shiner perch, topsmelt, surfperch, mosquito fish, flat croaker, creek chub, sand shiner, plains minnow, gizzard shad, river carpsucker, and river shiner. During the nesting season, adults often carry small fish long distances to feed young or mates.

Nesting

Nest Placement

Although both sexes make several nest scrapes, the female choses the nest site itself, often near nests of other species such as Black Skimmers or Piping Plovers. The nest site is usually well drained and not far from foraging habitat.

Nest Description

The nest is simply a scrape in the sand or other substrate, sometimes with pebbles, shells, or bits of vegetation added. The scrape typically measures about 4 inches across by 0.8 inches deep.

Nesting Facts

Behavior

Least Terns nest in colonies. Pairs are monogamous and often stay together over multiple nesting seasons. Pairs form or renew their bonds in spring on the nesting grounds or on “courting grounds” that can sometimes be quite far from the colony site.

Males defend preferred display sites, often on logs or some other elevated perch, and females likewise choose a favored location to await courtship. Males bring a small fish back to a group of terns, calling, and several terns then follow him in the air, gliding and then landing with stiff wings. The male then presents the fish to the female, who usually consumes it. This courtship feeding continues well into the nesting period and during incubation. Pairs also perform courtship displays while on the ground, raising and lowering bills and bodies, much like larger tern species.

Females select the nest site, and both sexes defend it, or at least a small space around it (perhaps 3–10 feet), by launching attack flights against intruders or predators and calling loudly, sometimes defecating. Both parents tend the chicks, taking turns foraging and tending them at the nest site. After chicks fledge, Least Terns gather into flocks for foraging and migration. Their flight is strong and direct, on stiff, jerky, rapid wingbeats.

Conservation

Least Tern numbers have declined severely in the last half-century. The species' habit of breeding in widely distributed colonies makes it difficult to monitor population trends with precision. Nevertheless, the North American Breeding Bird Survey estimates Least Tern numbers declined 4.13% per year from 1966 to 2015—equivalent to a cumulative decline of about 87% over that period. Partners in Flight estimates a global breeding population of 120,000 birds, of which 52,000 breed in the U.S. and Canada. The group rates Least Tern a 15 out of 20 on the Continental Concern Score and includes it on the Yellow Watch List for species with declining populations. Least Terns are locally common, but their favored nesting habitats (sandy beaches) are also prized for human recreation, development, and alteration by water diversion—all activities that interfere with successful nesting. The "interior" subspecies (athalassos) was federally listed as Endangered from 1985–2021, during which time conservation workers were able to grow the population from under 2,000 individuals to more than 18,000. Oil spills in the Gulf of Mexico have had negative impacts on Least Terns; the species is also vulnerable to other pollutants that affect water quality or impact the fish they eat. Changes in prey distribution and availability, and loss of coastal nesting areas and wintering areas to sea-level rise, are among the problems predicted for this species in the era 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. Longevity records of North American birds. Version 2021.1. Patuxent Wildlife Research Center, Bird Banding Laboratory 2021.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

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, NY, USA.

Thompson, Bruce C., Jerome A. Jackson, Joanna Burger, Laura A. Hill, Eileen M. Kirsch and Jonathan L. Atwood. (1997). Least Tern (Sternula antillarum), 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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