Habitat

Usually forms colonies on bare or sparsely vegetated sand or dried mudflats along coasts or rivers, but also on sandy or shell islands and gravel and sand pits (Ganier 1930, Downing 1973, Massey 1974, Jackson 1976, Burger 1984, Sidle et al. 1988, Burger and Gochfeld 1990a, Smith and Renken 1991). Occasionally nests among stones (Fletcher 1980). Prominent use of sand or shell beaches located just above high-tide level swept free of vegetation by periodic, high storm tides, or riverine sandbars on which vegetation is limited by seasonal flooding (Hardy 1957, Tomkins 1959, Massey 1974, Wolk 1974, Burger 1984). Colonies generally near lagoon, estuary, river, or coast. Often nests on deposited dredged materials (Thompson and Slack 1982, Gochfeld 1983, Hovis and Robson 1989, Burger and Gochfeld 1990a). On Long I., NY, most frequently used habitats include peninsulas, barrier islands, and sandy shorelines (MacLean et al. 1991). Also nests on flat gravel roofs in Florida (first record in 1957) and Mississippi (Fisk 1975, 1978, Jackson and Jackson 1985, Gore and Kinnison 1991).

Colony habitats sometimes ephemeral, presenting problems for continued use, but species shows high colony site tenacity (Burger 1984, Kirsch 1996) and fidelity (Atwood and Massey 1988). Colony site tenacity influenced by changes in vegetation cover, predators, human activities, floods, and colony size (Kotliar and Burger 1986, Atwood and Massey 1988). Colony sites limited by presence of suitable nest sites (interior race; Sidle and Kirsch 1993) or human activities (coastal; Burger 1984, 1989). Successful colonies of various sizes usually reoccupied; small colonies tend to be less stable (Gochfeld 1973, Burger 1984).

Nesting habitats have changed during twentieth century, especially in California and Gulf of Mexico. Beach habitats increasingly used for human recreation and residential development; river sandbars destroyed by channelization, water diversions, impoundments, or by changes in vegetation resulting from controlled water flow below dams. Agricultural fields, parking lots, bare land associated with airports, and flat, graveled rooftops have provided occasional alternative nesting habitats. Traditional habitats often abandoned while human-made habitats have been colonized. As early as 1908, California Least Terns nested on dredged material covering a former marsh (Chambers 1908), but in early 1980s in California, colony sites were natural substrate (15 of 27 sites) where available (Erickson 1985). In N. Carolina, 72% of Least Tern nests were on dredged-material islands (Jernigan et al. 1978). In Mississippi (1970s and 1980s), >90% of nests in human-made habitats such as dredged-material islands, constructed beaches, building sites, and graveled rooftops (Jackson and Jackson 1985). In New York, shifted from beach habitats to sandy knobs and broad sand flats (Gochfeld 1983). In S. Carolina before 1960: 4 site types, and 83% of nests on traditional sand beaches or shell banks; since 1960, terns have used 7 site types, and 56% of nests on traditional sites (Savereno and Murphy 1995). In Nebraska, where central Platte River no longer provides suitable habitat, because of diversion of water upstream, Least Terns nest at commercial sand and gravel pits within 1.5 km of the Platte (Sidle and Kirsch 1993). In Iowa, species has nested on fly ash effluent at power plants (Dinsmore et al. 1993, Huser 1996).

Human-wrought habitat changes have at times allowed species to occupy new areas, but negative factors such as excessive heat and domestic pets can markedly reduce productivity, and addition of these new habitats is minimal compared to the amount of natural habitat lost to human activities. Habitats capable of supporting successful nesting require adequate supply of accessible prey species nearby. Large reservoirs and aquaculture operations provide attractive foraging habitat. Prey accessibility may be limited by turbidity in silt-laden rivers and streams or by choppy water caused by frequent boat or jet ski operation.

Migrates along Pacific, Atlantic, and Gulf Coasts or major river drainages in U.S. to coastal habitats along Baja California, s. Mexico, and South America. Often migrates well offshore (>25 km) near Panama (Ridgely 1976), and common fall migrant through Costa Rica (Stiles and Skutch 1989). Least terns seen 2–30 km (most <18 km) off western coast of Baja California in late Apr and early May, associated with feeding flocks of other seabirds (Howell and Engel 1993).

Little known of winter habitats. Primarily along marine coasts in littoral zone and in bays and estuaries; sometimes concentrates at mouths of rivers (ffrench 1991). In Colombia, in coastal lagoons and mudflats, rarely inland over fresh water (Hilty et al. 1986). In Suriname, wintering Least Terns were reported feeding at inshore waters and estuaries of large rivers, along creeks, exposed mudflats, and at shallow lagoons (Spaans 1978). The similar Little Tern has been reported up to 600 km off coast of w. Africa (Cramp 1985); thus, winter pelagic feeding possible in Least Tern.