Going the Distance

Question: What part-time resident of Iceland is the record holder for having the longest annual migration of any creature on Earth?

Answer: Arctic tern

If you’re always complaining about your lengthy commute, don’t expect any sympathy from an Arctic tern. Year in and year out, these streamlined seabirds with blue-white bodies, forked tails, and black hoods fly from Iceland to Antarctica and back, covering a distance of some 50,000 miles. Over the course of its three-decade lifespan, the bird—weighing just 3.5 ounces—logs about 1.5 million miles, the equivalent of three round-trip flights to the moon. That’s an awful lot of frequent flyer miles.

A lot of birds and animals migrate, of course. It’s just that Arctic terns take it to the extreme. So, what do they get out of all that flying?

I’ll follow the Sun 

Just like the classic Beatles tune, Arctic terns (Sterna paradisaea) follow the summer sun from the Arctic Circle all the way to the Antarctic Circle. It’s a lot of work, but the payoff is huge—it gets them two entire summers a year and more daylight than any other animal on Earth. It’s not like people from Boston who head to Florida to escape the winter cold, either. Instead of seeking warmth, Arctic terns are strictly seeking the summer sunlight. As you’re probably aware, summers in the Arctic and Antarctic get almost 24 hours of sunlight, and during the winters it’s almost entirely dark.

What makes sunlight so important for Arctic terns and other marine birds is how it illuminates the ground and ocean surface, making it much easier to find fish or insects. The summer weather also means calmer seas that make for easier flying. As difficult as it is to make a pole-to-pole migration, it would be even more difficult to find food in the dark Arctic winter.

So, why doesn’t everyone use this survival strategy and follow the sun? If it were easy, they would. Unlike most seabirds, Arctic terns are built for long-distance migration. And they’ve evolved over millions of years to undertake such an arduous journey with relative ease. They are so lightweight, they can just let the ocean breezes carry them great distances without having to flap their wings—which uses up lots of energy. Like switching to auto-pilot, the birds spend most of their time gliding—even when they sleep and eat. In fact, just like hummingbirds, Arctic terns are one of the few birds who can hover in midair. That gives them the ability to stay still in the air while they scout for fish or predators before taking a deep dive towards the water or ground—something no other Icelandic bird can do.

Tern! Tern! Tern! 

To keep the musical theme going, that hit song by The Byrds (!) can serve as a reminder of the massive population of Arctic terns who spend their summer in Iceland. Although their numbers are declining, there are thought to be more than 250,000 breeding pairs of Arctic terns in Iceland, about 30% of the worldwide population. They breed in colonies of several thousands of pairs scattered in the flat grasslands of Iceland’s coastal areas.

While it is a joy to observe these elegant and graceful birds in flight, they turn feisty and aggressive in a hurry when anyone gets too close to their nesting grounds. They will hover and screech as a warning, but those who don’t back off immediately can expect a nose-dive to the head that could easily draw blood. In fact, they’re so fiercely protective, even predators stay away and other birds try to build their nests close by (like they’re moving into a gated community for the safety factor). Icelanders always respect the nesting sites of Arctic terns (which they refer to as Kríuvarp) and know to back away if they accidentally stumble upon a colony during nesting season—before it turns into a scene from a classic Hitchcock film you may recall.

What they are protecting, of course, are the babies. About three weeks after Arctic terns arrive back in Iceland from Antarctica in mid-May, they will welcome two or three hatchlings into the family. Now their focus is feeding them for the three or four weeks it takes until they can fly on their own. Krill and insects will do, but the go-to food in Iceland is the sand eel, which they’ll scoop up to feed the juveniles. The thing is, they can only fit one or two of those tiny eels in their beaks at a time, so they have to make numerous foraging trips. (Contrast that with another migratory seabird, the puffin, who also relies on sand eels to feed both themselves and their growing chicks but can hold dozens of them in their beaks at the same time—in fact, the record is 83 sand eels held by a single puffin. If you’re familiar with competitive eating, he could be the Joey Chestnut of the puffin world.)

Baby, it’s cold outside 

When the season ends, temperatures drop, and the polar light fades, Arctic terns are ready to leave their breeding grounds across the circumpolar Arctic and head south. They all migrate together and when it’s about to take place, the normally raucous colony falls eerily silent—a behavior that in scientific circles is called, appropriately enough, “dread.” Then, when dread is over, they all take to the sky at once and chase the sun. Their journey takes them southwards via the Atlantic, past the western coast of Africa, and down into the Southern Ocean surrounding Antarctica.

You may have noticed how we claimed Arctic terns fly some 50,000 miles round trip—and they do, as it has been confirmed with tracking devices. But the distance from Iceland to Antarctica isn’t 25,000 miles; it’s closer to 18,000 miles. It turns out the terns are in no great hurry to get there. Rather than flying in a straight line, they go thousands of miles out of their way to take advantage of the best weather conditions and food sources. After chillin’ for a few months in Antarctica, they’ll do it all over again, returning to Iceland and the Arctic the following spring.

Some animals stay in the same place all year around, while others cover great distances—by walking, swimming, or flying—every spring and fall. In appearance and behavior, Arctic terns aren’t so very different than many other marine birds. But their extraordinary annual migration makes them true superstars in the bird world, and in the entire animal kingdom as well.

A few more migration superstars of the animal world

• Pick up the pace, Rudolph—Of the land-bound mammals, the North American caribou—better known as reindeer—are impressive migraters, traveling up to 3,000 miles each year. The herds migrate north in the summer to feast on the blossoming tundra and then turn back in the winter, when the snow and ice return.
• Strength in numbers—One of the largest, and most dramatic, mammal migrations in the world is Africa’s annual wildebeest migration. A herd of 1.5 million wildebeest, accompanied by hundreds of thousands of zebra and gazelles, travel close to 1,000 miles on a looping path through Kenya and Tanzania following the seasonal rains. Their large numbers help protect against predators while they consume some 7,000 tons of phosphorous-rich grass along the way.
• Buggin’ out—Dragonflies have a spot in the record book for embarking on the longest of all insect migration paths. While much of it is still shrouded in mystery, a scientist in 2009 observed dragonflies following the monsoon rains from India to Africa and back, a journey that covered a distance of 10,000 miles and involved a couple of generations of dragonflies.
• Migratory royalty—Monarch butterflies may not set any migrating distance records, but they’d  certainly win a beauty contest. Just like birds, huge swarms of the orange-winged North American monarchs travel south for some 3,000 miles for the winter, using air currents and Earth’s magnetic field to guide them as far as Mexico.
• It has its ups and downs—You may be surprised to learn that zooplankton (like smaller fish, shrimp, jellyfish, etc.) represent the greatest migration on Earth. In something called “diel vertical migration” (DVM), billions upon billions of zooplankton migrate up from the ocean depths each night and then back down again when it gets light—by far the largest movement of biomass on Earth.
• Santa claws—Christmas Island in the Indian Ocean is home to an annual migration of some 120 million red crabs. Each year between October and December, the bright red crustaceans emerge from their rainforest homes and trek across the island towards the ocean to mate—unfortunately, about a million of them get crushed by cars and trains along the way.
• Thar she goes—While tracking an endangered subspecies of gray whales In 2015, marine biologists recorded a 9-year-old female who swam across the Pacific from Sakhalin Island in Russia to Baja, Mexico and back—a distance of 14,000 miles in 172 days. It is the longest mammal migration in history.
• Smoked in the end—Salmon deserve recognition just for the extra effort they put in during migration. Born in freshwater, they live in the sea until it’s time to reproduce. Then, they travel thousands of miles using Earth’s magnetic field to find the stream where they were born, fighting their way through seawater and freshwater and swimming upstream to the precise location. And the reward for doing their part to continue the species? They die...
• Over the hump—Found in oceans around the world, many humpback whales embark on an annual migration stretching more than 5,000 miles, swimming at speeds between three and nine miles per hour.
• Soup’s on!—Sea turtles do an awful lot of moving around during their lengthy lives. Leatherback sea turtles in particular travel more than 10,000 miles across the Pacific each year and are able to make it back to the very beach where they were born.
• Dude, surf’s up—Birds and animals may do most of the seasonal migrating, but sometimes people migrate, too. The 1966 documentary, The Endless Summer, is about two surfers who travel around the world following the summer season in search of the perfect wave.
• Tipping our hats to Arctic terns—Millions of arctic terns were hunted in North America and Europe during the nineteenth century so their feathers could be used to decorate women’s hats. Let’s hope the birds managed to bloody a few hunters’ scalps first ...

Learn about the migratory habits of all the arctic flora and fauna of Iceland on our Untamed Iceland adventure.