The arctic tern (Sterna paradisaea) is a water-loving bird that hatches during summer in the Arctic Circlethe northernmost part of the Northern Hemisphere. During the extremely colddark arctic winterthe arctic tern flies southfollowing the summer season all the way to the Antarctic Circle on the other side of Earth.

Because arctic terns do not fly in a straight linethe distance they fly every year is even longer than the approximately 30,000-kilometer (18,641-mile) from Arctic Circle to Antarctic Circle. This makes the arctic tern’s migration one of the longest of any animal on Earth.

Bird Behavior

Like a lot of other birdsarctic terns eat fish. They catch fish by gliding over the oceanthen plunging their feet or beaks in the water to skim fish near the surface. Unlike pelicans or ducksarctic terns are not good swimmers and don’t spend a lot of time in the water. Rarelyarctic terns will snatch flies or other insects out of the airbut they prefer fish and other marine creaturessuch as shrimp.

Arctic terns have beaks that are almost the same shade of tomato-red as their webbed feet. They have gray-white bodies and a head of jet-black featherswhich looks almost like a baseball cap.

Arctic ternswhich mate for lifecan live to be more than 30 years old. This is a very long lifespan for such a small bird with such an extreme life.

A group of arctic terns is called a colony. A tern colony migrates together. Just as migration is about to take placethe normally noisy colony will fall silent. This behavior is called dread. After dreadthe colony will take to the air and leave their home nests all at once.

It might be easy to scoff at how normal the arctic tern might seem. Their dietappearanceand behavior are similar to other marine birds. But their extraordinary yearly wandering puts them in their own category in the bird world.

Why Migrate?

Arctic terns migrate to follow the summer sun. Seasons happen because Earth is tilted on its axis while it revolves around the sun. During winterthe Northern Hemisphere is tilted away from the sun’s warming rays. This is why it’s colder during the winter in places north of the Equatorlike the United States.

When it’s winter in the Northern Hemispherethe Southern Hemisphere is tilted toward the sunexperiencing summer. In Decemberwhen people in the United States are putting on sweaters and jacketspeople in Argentina are getting out their sunscreen.

Like areas near the Equatorpolar regions experience less noticeable temperature changes than other parts of the globe. Howeverthese regions experience a great difference in the amount of daylight hours. During summerthe Arctic and the Antarctic get almost 24 hours of sunlight. During winterit is almost entirely dark.

The arctic terngoing from Arctic summer to Antarctic summermay experience more daylight than any other animal. Terns migrate in search of summer sunlight. Sunlight illuminates the ground and the ocean surfaceso the birds can see fish or insects more clearly. Summer weather is also usually calmer at seaallowing the birds to fly more easily.

“It’s really difficult to migrate this farfor this long. But it’s even tougher to find food in the Arctic winter,” said Doug Inkleysenior scientist with the National Wildlife Federation.

"It’s a strategy for survival," Inkley said.

Greg Butcher of the National Audubon Society said"If it weren’t so hard to doeveryone would do it,” concerning their constant pursuit of summer.

Evolving to Migrate

Over millions of yearsarctic terns evolved to undertake their unique migration.

"They didn’t just get up one day and say‘GeeI think I’ll fly to Antarctica,’" Inkley said.

Arctic terns are made for migration. They prefer to glide in the air for most of the year. They are so lightweightthey let ocean breezes carry them great distances without having to use a lot of energy flapping their wings. Arctic terns can sleep and eatall while gliding. In factarctic terns are one of the few birdsbesides hummingbirdsthat can hover in midair.

"They could fly 1,000 miles [1,610 kilometers] a day if they didn't need to fuel up in between," Butcher said.

After fitting the birds with trackersscientists learned that arctic terns fly thousands of miles out of their way to take advantage of the best weather and get the best food. They can bounce around every continent instead of flying in a straight line back home. Although most arctic terns return to their home nesting groundssome birds veer off course. Arctic terns from Siberia have shown up in South Africawhile terns that hatched in Greenland have been sighted in Australia.

The whole journey only takes the terns a couple of months.

Other birds have pretty long migrationsincluding the barn swallow (Hirundo rustica) and the sooty shearwater (Ardenna grisea). The sooty shearwaterin factgoes almost as far as the arctic tern. Stillno other animal makes a commute like this.

You might think that such a small bird would get snapped up by predatorsbut that’s not the case. Arctic terns are not endangeredButcher said. Their breeding grounds are in the high Arctic: the coldestmost remote part of the region. This makes their nests hard to find. Even arctic foxes (Vulpes lagopus)the birds’ main predatorhave trouble finding themButcher said.