Least terns can be distinguished from all other terns by their small size and combination of a black crown and eye stripe with a white forehead. The yellow legs of mature birds are relatively short, with webbed to partially webbed feet. First-summer birds are distinguishable by their dark bills and legs, with a black line though the eye, and a dark shoulder bar. Least Terns eat mainly fish, although invertebrates such as crustaceans and insects are also taken. They fish in shallow water, very close to shore, hovering longer than other terns and plunging to the surface, without submerging, to capture their prey.
Nests are shallow scrapes in the sand, soil or gravel, and sometimes under low bushes.
Both males and females construct several scrapes, which are formed by sitting on the substrate and kicking their feet while rotating their bodies. Ultimately, the female chooses the scrape that will become the nest. There is great variation within clutches in respect to the number, size, and spotting of the eggs. Although both parents care for the eggs during the incubation period, females do more than their share. During excessively hot weather, parents may soak their bellies in water and drip it onto the eggs, or chicks, as a method of cooling them.