Terns are seabird

Seabird

Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

s in the family Sternidae, previously considered a subfamily (Sterninae) of the gull family Laridae (van Tuinen et al., 2004). They form a lineage with the gulls and skimmer

Skimmer

The Skimmers, Rynchopidae, are a small family of tern-like birds in the order Charadriiformes, which also includes the waders, gulls and auks. The family comprises three species found in South Asia, Africa, and the Americas....

s which in turn is related to skua

Skua

The skuas are a group of seabirds with about seven species forming the family Stercorariidae and the genus Stercorarius. The three smaller skuas are called jaegers in North America....

s and auk

Auk

An auk is a bird of the family Alcidae in the order Charadriiformes. Auks are superficially similar to penguins due to their black-and-white colours, their upright posture and some of their habits...

s. Terns have a worldwide distribution.

Most terns were formerly treated as belonging to one large genus Sterna

Sterna

Sterna is a genus of terns in the bird family Sternidae. It used to encompass most "white" terns indiscriminately, but mtDNA sequence comparisons have recently determined that this arrangement is paraphyletic...

, with the other genera being small. However analysis of DNA sequence

DNA sequence

The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

s supports the splitting of Sterna into several smaller genera (see list, below) (del Hoyo et al., 1996; Bridge et al., 2005; Collinson 2006).
Many terns breeding in temperate zones are long-distance migrants

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

, and the Arctic Tern

Arctic Tern

The Arctic Tern is a seabird of the tern family Sternidae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America...

 probably sees more daylight than any other creature, as it migrates from its northern breeding grounds to Antarctic waters.