Terns are seabirds and have a worldwide distribution. Many terns breeding in temperate zones are long-distance migrants, and the Arctic Tern probably sees more daylight than any other creature, as it migrates from its northern breeding grounds to Antarctic waters.
They are, in general, medium to large birds, typically with grey or white plumage, often with black markings on the head. They have longish bills and webbed feet. They are lighter bodied and more streamlined than gulls, and look elegant in flight with long tails and long narrow wings. Terns have deeply forked tails OR shallowly forked tails OR have unusual 'notched wedge' shaped tails. Terns ranges in size from the Least Tern [23 cm], to the Caspian Tern [21 inches].
Most terns hunt fish by diving, often hovering first, but the marsh terns pick insects of the surface of fresh water. Terns only glide infrequently; a few species, notably Sooty Tern, will soar high above the sea. Apart from bathing, they only rarely swim, despite having webbed feet.
Terns generally nest in large, densely packed colonies. Depending on the species and habitat, the nests may consist of unlined scrapes in the ground, or of flimsy collections of sticks on trees or floating vegetation. Terns are generally long-lived birds, with several species now known to live in excess of 25–30 years.