Birds
Arctic Tern
Sterna paradisaea · also called sea swallow
The Arctic tern is a small, slender, grey-and-white seabird that lives its life chasing the sun. Every year it flies from one end of the planet to the other and back, the longest migration of any animal, so that it spends almost all its time in summer daylight.
Overview
Arctic terns breed in the short northern summer on coasts, islands, and tundra right around the Arctic and sub-Arctic — from Alaska and Arctic Canada across Greenland, Iceland, and northern Europe to Siberia. As the northern autumn closes in, they head south to the Antarctic, spending the northern winter (the southern summer) foraging along the edge of the pack ice and across the Southern Ocean, before turning around and flying back to breed. A tern is at one pole or the other, or travelling between them, essentially all the time.
The longest migration on Earth
Geolocators light enough to fit on a tern have revealed the true scale of the journey: a round trip of 70,000 to 90,000 kilometres a year. The route is not direct — the birds ride prevailing winds in enormous looping detours down the Atlantic and around the Southern Ocean — so the actual distance flown is four or five times the straight-line pole-to-pole distance. Over a lifespan that can exceed 30 years, an Arctic tern flies well over two million kilometres.
A side effect of this lifestyle is that the Arctic tern sees more daylight than any other animal, following the midnight-sun period at both ends of the world.
Feeding and flight
Arctic terns feed on small fish and crustaceans taken from at or just below the sea surface, either by dipping in flight or by a shallow plunge-dive. Their long wings and light build make them efficient gliders, and they feed as they travel, so the migration doubles as an extended fishing trip. They can rest on the water or on ice floes.
Reproduction
At the breeding colony, pairs perform a “fish flight” courtship and nest in a bare scrape on open ground. They lay one to three camouflaged eggs and defend the nest with ferocious aerial attacks on anything that comes near. Chicks fledge in about three to four weeks, and the family may migrate south together, the young making their first pole-to-pole journey within weeks of learning to fly.
Status
The Arctic tern is listed as Least Concern with a global population around two million, but it is declining in parts of the North Atlantic. The main problem is food supply at the breeding grounds — warming seas and industrial fishing have reduced sand eels and other small fish, causing widespread breeding failure in colonies in Britain, Iceland, and elsewhere. Its Antarctic wintering grounds are also affected by changes in sea ice and krill. As a species that depends on the health of the ocean at both ends of the Earth, it is considered a sensitive indicator of global marine conditions.
Arctic Tern: common questions
How far does an Arctic tern migrate?
Farther than any other animal. Tracking studies fitting tiny geolocators to individual terns found round-trip journeys of 70,000 to 90,000 km per year between breeding grounds in the Arctic and wintering areas around Antarctica. The birds do not fly in a straight line — they follow the wind in long S-shaped detours across the Atlantic and around the Southern Ocean — which is why the distance is so much greater than the roughly 19,000 km straight-line pole-to-pole gap. Over a 30-year lifetime a tern may cover 2.4 million km.
Why does the Arctic tern travel so far?
To live in perpetual summer and abundant food. By breeding in the Arctic during the northern summer and then flying to the Antarctic for the southern summer, the tern spends the productive, food-rich season at each end of the Earth and largely skips winter altogether. It also means the Arctic tern is bathed in more daylight over the course of a year than any other animal, since it tracks the near-24-hour daylight of both polar summers.
How does such a small bird survive that journey?
Arctic terns are extraordinarily efficient flyers. They have long, narrow wings and light bodies, and they glide and use the wind rather than flapping constantly, feeding on the move by snatching fish and krill from the sea surface. They can rest on the water and on floating ice. The migration takes a few months in each direction with feeding stops, not a single non-stop effort, which spreads the energetic cost.
Are Arctic terns aggressive?
At the nest, yes — famously so. Arctic terns nest in colonies on open ground, and they defend them fiercely, diving at and striking intruders (including humans, foxes, and much larger birds) on the head with their sharp bills, drawing blood, while screaming and defecating on them. This collective "mobbing" is effective enough that other birds, such as eiders, often nest inside tern colonies for the protection.