Animal World Records

Longest Migration

The great salmon run of North America. The famous wildebeest trek across the Serengeti. The mammoth migration of the green sea turtle to lay its eggs. These are just a few examples of epic journeys undertaken by animals. But even these pale in comparison to the migration of a small, slender, white seabird with a forked tail that weighs less than half a pound: the Arctic tern.

This little bird truly is a global traveller. It is the only species of bird to routinely occur on each of the Earth’s seven continents. They breed primarily in the Arctic Circle during the brief summer, but after raising chicks they leave the cold north and fly south. Because of their slender shape and long tail streamers, Arctic terns are sometimes fondly referred to as ‘sea swallows’ – but, unlike true swallows, these birds don’t just drop down to southern Europe or Africa to spend the winter. No, they go much further than that. About as far as it is possible to go without coming back on themselves, in fact. They travel from the extreme north of the world to the extreme south: Antarctica, on the opposite pole. It turns out the Arctic tern isn’t just a long-distance migratory bird. It is the most long-distance migratory bird.

This incredible journey makes the Arctic tern’s name something of a misnomer because it spends less than half of its time in the Arctic – the rest of its time is spent along the Antarctic coast and flying in between the two. It could equally be called the Antarctic tern, although there is already another, closely related and very similar species called that, which ranges throughout the southern oceans. Perhaps ‘bipolar tern’ would be a more geographically accurate name for this bird.

It could be said that the Arctic tern ‘overwinters’ along the Antarctic coast, although by the time it reaches its destination, it will be pretty much summer there as well. Since it experiences two summers a year, and because the sun scarcely dips below the horizon during the polar summers, the Arctic tern may not necessarily experience a wealth of gloriously high temperatures, but it – along with any parasites it might inadvertently carry in its plumage – does see more daylight each year than any other animal on the planet.

And that’s not the tern’s only claim to fame. Pole to pole, the journey it undertakes is at least 19,000 km – and that’s assuming that it travels in a straight line, which it never does. It often takes a convoluted, meandering route, following the shape of the continents and taking advantage of prevailing winds.

After spending the southern summer in the Antarctic, during which it may circle the entire continent, the tern heads back to the Arctic, once more making the immense journey. The round trip may cover more than 70,000 km, by far the longest and greatest animal migration on Earth. Since some terns can live to be 30 years of age, they can fly a staggering 2.5 million kilometres during their lives – that’s the equivalent of travelling to the moon and back three times!

However, the migration of the Arctic tern is not a non-stop one. Since it is a seabird, it can occasionally rest on calm waters and even fish. The longest migration without a single pit stop is made by an unassuming mottled brown wading bird called the bar-tailed godwit. It travels from its breeding grounds in Alaska and Siberia to its wintering grounds in New Zealand – an epic journey of 11,000 km that takes a week or more. Because it cannot stop and feed en route, a godwit must fatten up beforehand, voraciously cramming itself with worms and crustaceans.

But the godwit must also make other preparations. In a process called autophagy, the bird consumes large portions of its own internal organs prior to its migration. Although its heart and flight muscles retain their mass, up to a quarter of certain organs, including its kidneys, liver, gizzard and intestine, are broken down. This process saves the energy that would otherwise be used in maintaining such organs, which, though normally vital, are almost useless during the trip. It also creates space for more fat, which is an efficient and long-lasting fuel. By shrinking their organs in this way, the birds shed about half of their non-fat weight before migrating. This means that although pre-flight godwits are much fatter than normal, they still weigh about 20% less than they do during the rest of the year, making the birds lighter and thus more efficient on the wing.

In 2007, a satellite-tagged female bar-tailed godwit known as E7 travelled non-stop for 11,680 km from her nesting grounds in Alaska, across the Pacific Ocean, all the way to the northern tip of New Zealand, in just eight days. She opted for a longer, more leisurely route for her return northwards, stopping off in the Solomon Islands and then the mudflats of Japan for refuelling, making a round trip of over 29,000 km.

For over a decade, E7’s initial journey from Alaska to New Zealand was considered the longest known journey without pausing to feed undertaken by any animal. But in September 2020, another bar-tailed godwit completing the same migration route beat that record. This male flew non-stop for 11 days, reaching his destination in a bay near Auckland in New Zealand after covering around 12,000 km. Surely, this is one of the most extraordinary physical tests undertaken by any of the world’s birds.