loon, common name for migratory aquatic birds found in fresh- and saltwater in the colder parts of the Northern Hemisphere. Its strange, laughing call carries for great distances. Like the grebes, loons float low in the water and their legs are placed far back. They are expert swimmers and divers, sometimes slipping below the surface to swim underwater, but they cannot walk on land and at nesting time must use bill and wings to inch along. In taking flight they patter across the water with their feet. Their long, sharp, strong beaks are well adapted for catching fish. North American species include the common loon, or great northern diver ( Gavia immer ), a black and white bird about 32 in. (80 cm) long; the red-throated loon ( G. stellata ); and the Arctic loon ( G. arctica ). Loons are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Gaviiformes, family Gaviidae.