Editor's Note: Study of the mitochondrial DNA of terns, along with their plumage characteristics, have suggested that the heretofore broadly defined genus Sterna is paraphyletic. Reclassification of this genus now places Least Tern in the genus Sternula. See the 47th Supplement to the AOU Check-list of North American Birds for details. Future revisions of this account will account for this change.
The Least Tern, very similar to the Old World Little Tern (Sternula albifrons), breeds widely along coastal beaches and major interior rivers of North America and winters broadly across marine coastlines of Central and South America. This is the smallest of an array of terns that nest on relatively open beaches and islands kept free of vegetation by natural scouring from tidal or river action. Although widespread and common in places, its favored nesting habitat is prized for human recreation, residential development, and alteration by water diversion, which interfere with successful nesting in many areas. Although adapted to shift breeding readily in response to sites that change within and among years, this tern appears to be most productive at colony sites that have endured for several years.
The Least Tern feeds mostly on small, shallow-bodied fresh- and saltwater fish, but its diet is varied and includes small crustaceans and insects. Before egg-laying, courtship is punctuated by elaborate rituals of aerial display and distinctive calling by males, after which the male offers fish to the female (see above photo). Least Terns nest in a simple scrape in sand, shell, or other fragmentary material throughout their breeding range; gravel rooftops and a variety of deposited materials have been used with varied success. A typical clutch is 2 or 3 eggs; both adults incubate and care for the young. This dainty tern is pugnacious when defending nest and young. Its well-known zwreep call of alarm identifies this tern long before it comes into view.
Once substantially reduced by collection to adorn women's hats, the Least Tern portrays a roller coaster of changes in population. Diminished by recreational, industrial, and residential development in coastal breeding areas and significantly altered hydrology at interior breeding areas since the 1950s, it is specially classified for protection in much of its North American range. No other wide-ranging North American tern has that unfortunate distinction.
The Least Tern has been widely and extensively studied in its U.S. breeding range. Detailed investigations include studies of basic biology, breeding behavior, and survivorship in California (Massey 1974, Massey and Atwood 1981); demographics, census, and subspecific distinction on the Gulf Coast (Thompson 1982, Jackson and Jackson 1985c, Thompson et al. 1992a); behavior, ecology, and site selection on the Atlantic Coast (Burger Burger 1984c, Burger 1988b, Burger 1989, Burger and Gochfeld 1990a); and habitat selection and productivity of interior populations (Sidle et al. 1988, Kirsch 1996). Despite the breadth of this and other research, the demography of this species is poorly known, and association of wintering areas with various breeding populations remains uncertain.