Laridae in the context of "Sooty tern"

Bird ringing (UK) or bird banding (US) is the attachment of a small, individually numbered metal or plastic tag to the leg or wing of a wild bird to enable individual identification. This helps in keeping track of the movements of the bird and its life history. It is common to take measurements and examine the conditions of feather moult, subcutaneous fat, age indications and sex during capture for ringing. The subsequent recapture, recovery, or observation of the bird can provide information on migration, longevity, mortality, population, territoriality, feeding behaviour, and other aspects that are studied by ornithologists. Other methods of marking birds may also be used to allow for field based identification that does not require capture.

The Arctic tern (Sterna paradisaea) is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south as Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast for the southern summer and back again about six months later. Recent studies have shown average annual round-trip lengths of about 70,900 km (38,300 nmi) for birds nesting in Iceland and Greenland, and about 48,700 km (26,300 nmi) for birds nesting in the Netherlands, while an individual from the Farne Islands in Northumberland with a light level geolocator tag covered a staggering 96,000 km (52,000 nmi) in ten months from the end of one breeding season to the start of the next. These are by far the longest migrations known in the animal kingdom.

Arctic terns are medium-sized birds. They have a length of 28–39 cm (11–15 in) and a wingspan of 65–77 cm (26–30 in), and weigh around 100 g. In the breeding season they are mainly grey and white plumaged, with a blood red beak and feet, a black crown and nape, and white cheeks. The mantle is grey, and the scapulars grey, tipped white. The upper wing is grey with a white leading edge and a narrow blackish trailing edge on the primary feathers; the collar is completely white, as is the rump. The deeply forked tail is whitish, with grey outer webs. In winter, the forehead becomes partly white, and the bill and legs are black. Juveniles also have a partly white forehead, and narrow pale orangey-brown fringes on the grey mantle feathers (unlike common tern, where the mantle feathers are more completely gingery-brown).