Arctic tern

Distribution

The arctic tern has a circumpolar distribution and is the most northern of the terns.

It is a common breeding species on islands and coastal mainland areas in both the North Pacific and North Atlantic south to 50 °N, as well as in the High Arctic. In the eastern Atlantic it breeds in Greenland and Iceland, the British Isles, the Baltic Sea, Norway (including Svalbard) and the Barents Sea region.

The species is monotypic with little geographical variation. The arctic tern is common along the coast of most of Svalbard, and it is found in greatest numbers on the western and northern parts of Spitsbergen. It breeds either as single pairs or, more commonly, in colonies of up to several hundred pairs.

Many arctic terns spend winter in the pack ice of the Antarctic Ocean, but some probably remain in waters off western and southern Africa. Travelling this long distance twice a year, and in addition undertaking extensive movements in the Antarctic Ocean during the austral summer, means that the arctic tern has the longest migration of all birds.

It returns to Svalbard in the last days of May or at the beginning of June and leaves again between the end of August and the middle of September.

General ecology

The arctic tern nests on beaches, on the tundra close to the sea, and on islets and skerries along the coast within the Arctic, where they are generally safe from terrestrial predators. They occupy near-shore waters as well as being pelagic outside the breeding season.

The main food of the arctic tern is small fish (predominantly polar cod), crustaceans, polychaetes and insects caught near the water surface. Most of the prey is taken in relatively shallow water along the shore. Birds breeding in the most northerly parts of the range, such as in Svalbard, seem to be less dependent on fish and more on crustacean prey.

Predators entering the colony area are attacked furiously by the terns and other species such as the common eider, the long-tailed duck and the sabine’s gull seek protection by nesting in arctic tern colonies.

The arctic fox, glaucous gull and arctic skua are important predators of arctic tern eggs and chicks and may have significant impacts on local breeding success. The nest-site fidelity of breeding terns depends to a great extent on their breeding success the previous breeding season.

The population in the Barents Sea is estimated to be 130,000 breeding pairs. The European population, numbering more than 500,000 breeding pairs, is thought to be stable.

Life history and reproduction

Arctic terns usually arrive at the nesting site while there is still snow on the ground. They establish territories and wait for the snow to melt on the tundra before the specific site for the nest is chosen.

In some years, when thawing is late, they may refrain from nesting. The nest is placed directly on the ground, usually on dry gravel, though sometimes on damp moss. It is occasionally lined with grass, small stones or bits of shell.

The one or two (occasionally three) eggs are olive-brown with brown-black streaks and blotches. The timing of egg-laying varies from locality to locality and from year to year, depending largely on the timing of snow melt, but usually starts toward the end of June. The eggs are incubated by both sexes for 21–22 days.

The newly-hatched young remain in the nest until they are about three days old and thereafter move about through the colony area. They are fed by the parents for about three weeks before they fledge.

Arctic terns often experience rain and snow during incubation or while the young are small. This can cause high mortality among the eggs and young. Arctic terns become sexually mature at three or four years of age.

Like other seabird species, they have long lives. The oldest bird recorded in Svalbard was banded (ringed) as a chick in Hornsund in 1963 and recaptured alive at the same location 21 years later.

Management status and monitoring

The sizes of – and trends in – arctic tern populations are difficult to assess because terns frequently change the nesting locations.

The proportion of the population attempting to breed also seems to vary from year to year, probably depending on food availability. The breeding population in Svalbard is probably less than 10,000 breeding pairs, but no reliable estimates are available. There is little information concerning the current population trend, but it is thought to be stable over recent decades.