The arctic tern (Sterna paradisaea) is a water-loving bird that hatches during summer in the Arctic Circle, the northernmost part of the Northern Hemisphere. During the extremely cold, dark arctic winter, the arctic tern flies south, following 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 line, the 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 birds, arctic terns eat fish. They catch fish by gliding over the ocean, then plunging their feet or beaks in the water to skim fish near the surface. Unlike pelicans or ducks, arctic terns are not good swimmers and don’t spend a lot of time in the water. Rarely, arctic terns will snatch flies or other insects out of the air, but they prefer fish and other marine creatures, such 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 feathers, which looks almost like a baseball cap.

Arctic terns, which mate for life, can live to be more than 30 years old. This is a very long lifespan for such a small bird with such an extreme lifestyle.

A group of arctic terns is called a colony. A tern colony migrates together. Just as migration is about to take place, the normally noisy colony will fall silent. This behavior is called dread. After dread, the 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 diet, appearance, and 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 winter, the 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 Equator, like the United States.

When it’s winter in the Northern Hemisphere, the Southern Hemisphere is tilted toward the sun, experiencing summer. In December, when people in the United States are putting on sweaters and jackets, people in Argentina are getting out their sunscreen.

Like areas near the Equator, polar regions experience less noticeable temperature changes than other parts of the globe. However, these regions experience a great difference in the amount of daylight hours. During summer, the Arctic and the Antarctic get almost 24 hours of sunlight. During winter, it is almost entirely dark.

The arctic tern, going from Arctic summer to Antarctic summer, may experience more daylight than any other animal. Terns migrate in search of summer sunlight. Sunlight illuminates the ground and the ocean surface, so the birds can see fish or insects more clearly. Summer weather is also usually calmer at sea, allowing the birds to fly more easily.

“It’s really difficult to migrate this far, for this long. But it’s even tougher to find food in the Arctic winter,” said Doug Inkley, senior 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 do, everyone would do it,” concerning their constant pursuit of summer.

Evolving to Migrate

Over millions of years, arctic terns evolved to undertake their unique migration.

"They didn’t just get up one day and say, ‘Gee, I 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 lightweight, they let ocean breezes carry them great distances without having to use a lot of energy flapping their wings. Arctic terns can sleep and eat, all while gliding. In fact, arctic terns are one of the few birds, besides hummingbirds, that 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 trackers, scientists 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 grounds, some birds veer off course. Arctic terns from Siberia have shown up in South Africa, while 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 migrations, including the barn swallow (Hirundo rustica) and the sooty shearwater (Ardenna grisea). The sooty shearwater, in fact, goes almost as far as the arctic tern. Still, no other animal makes a commute like this.

You might think that such a small bird would get snapped up by predators, but that’s not the case. Arctic terns are not endangered, Butcher said. Their breeding grounds are in the high Arctic: the coldest, most remote part of the region. This makes their nests hard to find. Even arctic foxes (Vulpes lagopus), the birds’ main predator, have trouble finding them, Butcher said.