Arctic tern

Sterna paradisaea is a tern of the family Laridae described by Pontoppidan in 1763 ^[1]. In Peru, SERNANP lists it under the name «gaviotín ártico» ^[2]. It nests in the north, in Arctic and temperate regions, and spends the austral summer in the sea-ice zone around Antarctica, so each year it makes the longest return migration known among animals ^[3]. Birds tagged at a colony in Alaska travelled down the Pacific coast and stopped off Ecuador and Peru on their way south ^[4]. The IUCN Red List classifies it as Least Concern ^[5].

It is a small tern: adults recaptured at a colony in northeast Greenland weighed on average 106 g in 2007 and 110 g in 2008 ^[6]. It is a very close relative of the common tern, Sterna hirundo, on morphological and molecular grounds, and the two species occasionally interbreed in the few areas where both nest ^[7].

Compared with the common tern, the adult has shorter legs and probably somewhat longer wings ^[7]. In young birds the feature that separates the two species is on the wing: in the Arctic tern the secondaries are the palest part of the wing, while in the common tern they are never paler than the median and greater coverts; the young Arctic tern also has a white rump and, when it first flies, sometimes shows some red on the legs or at the base of the bill ^[7].

Similar species

• Sterna hirundo: The common tern has longer legs; in the young bird its secondaries are never paler than the coverts and its rump is grey, not white ^[7].

It breeds in the far north, in Arctic and temperate zones, and spends the boreal winter in the drifting pack-ice belt around Antarctica ^[3]. It travels south along two main routes: one through the eastern Atlantic, used by birds from Europe and Greenland, and another through the eastern Pacific, along the coasts of North and South America, probably used by birds from eastern Siberia and Alaska ^[3].

On the Pacific route, 6 terns tagged at a colony in Prince William Sound, Alaska, first flew about 2500 km to the California Current and then about 7000 km more to the northern part of the Humboldt Current, off Ecuador and Peru, between 5° N and 18° S ^[4]. Three of them went on another 2700 km to Chilean waters, between 28° S and 41° S, and from there moved to the Atlantic off Argentina before heading on towards Antarctica ^[4].

In the Peru Current it has been recorded from ships: during 14 cruises made between 1980 and 1995 in waters less than 370 km from the South American coast, between 3° S and 50° S, 17 Arctic terns were counted, classed as migrants that breed in the northern hemisphere ^[8].

Tracking with geolocators confirmed that its migration is the longest known among animals: 11 birds from Greenland and Iceland covered on average 70,900 km in a year, between 59,500 and 81,600 km depending on the individual ^[6]. For the 10 Greenland birds, the journey south took about 93 days and the return north only about 40, although the route back was only somewhat shorter than the route out; since the species can live more than 30 years, one bird could add up more than 2.4 million km over its life ^[6].

The Greenland birds made a stopover of about 25 days in the North Atlantic before going on south, and south of the Cape Verde Islands, of the 11 tracked birds, 7 followed the coast of Africa while 4 crossed towards that of Brazil ^[6]. They spent about 149 days in the Southern Ocean, south of 58° S, and went back north along an S-shaped route that circles the great gyres of the Atlantic ^[6].

The Alaska birds left the colony between late July and mid August 2007, and the only one whose logger worked all year was back in the breeding area by about 1 May 2008 ^[4]. On their way from Chile to Argentina, 5 of 6 birds crossed the Andes and Patagonia between 1 October and 6 November; those with reliable positions covered between 927 and 1286 km from one ocean to the other, and the crossing lasted between 14.3 and 35.5 hours; the sixth kept to the Pacific down to about 60° S ^[9].

In Antarctica, according to the classic hypothesis reviewed by the tracking studies, adults moult their flight feathers over about 60 days and spend much time resting on the ice ^[3]. They nest in colonies that can be small: in an archipelago in southern Sweden between 20 and 45 pairs breed each year, most of them on sandy islets ^[3].

Its route strings together very productive stretches of sea that act as refuelling stops: for the Alaska birds, ocean productivity was higher where they stopped than along the stretches where they only travelled, and at those stops they spent more hours a day looking for food ^[4]. They fed almost only in daylight, and over the whole journey they never had days with fewer than 12.5 hours of light ^[4].

Off Peru, the coastal upwelling that feeds the stop in the northern Humboldt Current runs all year round between 4° S and 16° S, although it varies from one year to the next ^[4]. Away from cold waters they are also seen at sea perched on floating logs or on turtles basking at the surface, and in the austral summer floating ice lets them rest out of the water ^[4].

The crossing of the Andes seems to be explained by the weather rather than by a lack of food: going round Cape Horn or through the Strait of Magellan would expose them to very strong winds, faster than their own flight speed ^[9]. According to the classic hypothesis, in Antarctica they make use of the abundant krill of the pack-ice belt, and the Baltic birds moved there at only about 60 km per day, probably while they moulted and fed ^[3].

The IUCN Red List classifies it as Least Concern ^[5]. In the fauna list of Los Manglares de Tumbes National Sanctuary it carries no category under D.S. N.° 004-2014-MINAGRI and is not in the CITES appendices ^[2].

Its reliance on a few stopover areas makes it sensitive to changes in those seas: those stops can run short of food during El Niño, and can also be affected by toxic algal blooms, pollution, changes in fisheries and climate change ^[4]. The greatest concern lies in Antarctica, because birds from several populations spend the austral summer on the same strip of the ice edge in the Weddell Sea, where krill has declined and a large loss of sea ice is expected ^[4].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

Because seabirds from populations around the globe gather in the same Antarctic waters during the austral summer, the authors of the Alaska tracking study warn that this mixing could influence how bird diseases evolve and spread, influenza A in particular ^[4].

It appears in the list of fauna species of Los Manglares de Tumbes National Sanctuary given in its Plan Maestro 2023-2027 ^[2]. The birds tagged in Alaska stopped off the coast of Ecuador and Peru in 2007: 4 of them were already there by late August, and the other 2 arrived later and stayed until the third week of October ^[4].

It was scarce in the seabird censuses of the Peru Current, off Peru and Chile: the ship-based surveys made between 1980 and 1995 counted only 17 individuals, against 210 South American terns, Sterna hirundinacea, and 168 Peruvian terns, Sterna lorata ^[8].