Nests solitarily or in small to large colonies (Ehrlich et al. 1988) on ocean and barrier beaches, as well as in salt marshes (Portnoy et al. 1988) and on sand bars (Scott 1983). Preferred habitat may be pebbles or shells on a sparsely vegetated sandy beach (Ehrlich et al. 1988) near extensive shallow waters for foraging (Kaufman 1996). Before eggs are laid, may roost elsewhere at night. Forages by diving (Ehrlich et al. 1988) or catching insects in mid-air. In other parts of the species' range, also found along rivers and lakes (Kaufman 1996).

Nests in open, flat areas, and occasionally on rooftops (Ehrlich et al. 1988).

NATURE SERVE GLOBAL HABITAT COMMENTS:

Seacoasts, beaches, bays, estuaries, lagoons, lakes, and rivers (AOU 1983). Rests and loafs on sandy beaches, mudflats, and salt-pond dikes (Stiles and Skutch 1989). In California, may roost at night on sandy beaches away from nesting areas for several weeks before nesting. In nonbreeding season (September-March) flocks have been found at sea, often far from land, in southeastern Caribbean and adjacent Atlantic off Guianas (van Halewyn and Norton 1984).

Nests usually in shallow depression on level ground on sandy or gravelly beaches and banks of rivers or lakes, typically in areas with sparse or no vegetation (usually less than 20% vegetation cover, often 10% or less; Bent 1921, Craig 1971, Jernigan et al. 1978, Thompson and Slack 1982, Faanes 1983, Gochfeld 1983, USFWS 1990); also on dredge spoils; on mainland or on barrier island beaches; also on flat gravel-covered rooftops of buildings (especially in the southeastern U.S.) or other similarly barren artificial sites (AOU 1983). Good nesting areas tend to be well beyond the high tide mark, have shell particles/stones/debris for egg camouflage (Burger and Gochfeld 1990), be out of the way of ORVs and the general public recreation areas, not subject to unusual predation pressure, and adjacent to plentiful sources of small fishes. Colonies on small islands usually experience less mammalian predation (Burger 1984). Good roof-top sites provide some shade for chicks.

Adults do not require cover during the breeding season, but chicks may use sparse vegetation and debris for shade and protection (Hardy 1957, Blodgett 1978). Parents may lead chicks toward the periphery of the colony into more heavily vegetated areas (Akers 1975), where the young utilize debris and vegetation for cover (Hardy 1957). In coastal areas, beach grass (AMMOPHILA BREVILIGULATA) is the commonly associated vegetation. Along river systems, willow (SALIX spp.) is the common vegetation adjacent to sites (Sidle, pers. comm. 1985). On Oklahoma salt flats, almost 60% of the nests were within 5 cm of debris (Grover and Knopf 1982).

Dredge spoil islands are often excellent locations for tern colonies, exhibiting habitat characteristics that attract least terns. However, the substrate composition of dredge spoil has presented problems in Texas. Natural sites largely consist of sand and shell fragments and less than 10% silt and clay. Most dredge-spoil deposition sites are composed of a mix of a variety of particles and greater than 45% silt and clay. The fine silts and clay in some dredge spoil sites sometimes promote 'egg sticking' which occurs during wet periods and causes egg loss. These 'artificial' substrates contain sufficient sand to stimulate terns to select the site for nesting, but the finer texture of the silt particles reduces drainage (Thompson and Slack 1982). Furthermore, dredge spoil sites are short-lived and typically undergo rapid succession (Burger 1984).

Interior populations nest mainly on riverine sandbars or salt flats that become exposed during periods of low water (Hardy 1957). As a result of vegetational succession and/or erosion, preferred nesting habitat typically is ephemeral. Hardy (1957) implied that breeding in riverine situations depends on the presence of sandbars, favorable water levels during nesting season, and sufficient food. Nests are usually located at higher elevations and away from the water. Water levels determine the size of sand bars and the extent of nesting areas (USFWS 1990). Dams above colonies generally lower habitat quality by eliminating the spring floods that are necessary for alluvium deposition and the scouring of vegetation. Ducey (1982) reported successful breeding at two privately-owned sand and gravel companies along the Platte River in Nebraska. As old breeding sites became unsuitable due to vegetation encroachment, the terns simply moved to more recently created sand deposits. See also Ziewitz et al. (1992) for information on nesting habitat in the Platte River in Nebraska. Populations in Kansas have nested on oil well sites (Schulenberg and Ptacek 1984).

Since least terns always nest near water, they are vulnerable to flood inundation and seem to seek high ground. In coastal Texas, Thompson and Slack (1982) documented that the densest nesting area in 67% of the colonies was above the midpoint of available elevations. Gochfeld (1983) found that least terns on Long Island avoid beaches that have less than 32.8 feet (10 m) of width beyond the hightide mark. Interior least tern nests on salt plains in Oklahoma were located an average of 110.5 m away from the nearest water (Grover and Knopf 1982). However, nests on the Platte River in Nebraska, were located at an average of 18.9 m away from the nearest river channel on sand bars that averaged 58.9 m wide (Faanes 1983).