Editing Arctic tern (section)

== Description and taxonomy ==
[[File:Juvenile Arctic Tern.jpg|thumb|Juvenile]]
The Arctic tern is a medium-sized bird around {{convert|33|-|36|cm|abbr=on}} from the tip of its beak to the tip of its tail. The wingspan is {{convert|76|-|85|cm|in|abbr=on}}.<ref name="HBW"/> The weight is {{convert|86|-|127|g|abbr=on}}. The [[beak]] is dark red, as are the short legs and webbed feet. Like most terns, the Arctic tern has [[bird flight|high aspect ratio wings]] and a tail with a deep fork.<ref name="HBW"/>

The adult [[plumage]] is grey above, with a black nape and crown and white cheeks. The upperwings are pale grey, with the area near the wingtip being translucent. The tail is white, and the underparts pale grey. Both sexes are similar in appearance. The winter plumage is similar, but the crown is whiter and the bills are darker.<ref name="HBW"/>

Juveniles differ from adults in their black bill and legs, "scaly" appearing wings, and mantle with dark feather tips, dark carpal wing bar, and short tail streamers.<ref name="HBW"/> During their first summer, juveniles also have a whiter forecrown.<ref name="National Geographic"/>

The species has a variety of [[bird call|calls]]; the two most common being the ''alarm call'', made when possible predators (such as humans or other [[mammal]]s) enter the colonies, and the ''advertising call''.<ref name="Hatch et al."/>

While the Arctic tern is similar to the [[common tern|common]] and [[roseate tern]]s, its colouring, profile, and call are slightly different. Compared to the common tern, it has a longer tail and mono-coloured bill, while the main differences from the roseate are its slightly darker colour and longer wings. The Arctic tern's call is more nasal and rasping than that of the common, and is easily distinguishable from that of the roseate.<ref name="Olsen"/>

This bird's closest relatives are a group of South Polar species, the [[South American tern|South American]] (''Sterna hirundinacea''), [[Kerguelen tern|Kerguelen]] (''S. virgata''), and [[Antarctic tern|Antarctic]] (''S. vittata'') terns.<ref name="Bridge et al."/>

The immature plumages of Arctic tern were originally described as separate species, ''Sterna portlandica'' and ''Sterna pikei''.<ref name=Kaufman1990/>