Planet Earth

Chasing daylight - tiny trackers reveal the incredible flight plans of the Arctic tern

Not Exactly Rocket ScienceBy Ed YongJan 14, 2010 10:07 PM


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I've just flown from London to North Carolina, a trip of around 6,200km. As flights go, it's a pathetic one, a mere jaunt in the park compared to the epic voyage of the Arctic tern. Every year, this greatest of animal travellers makes a 70,000 km round-trip, in a relentless, globe-trotting pursuit of daylight. In summer, it spends its time in the sun-soaked Arctic and in winter, it heads for the equally bright climes of Antarctica. In its 30 years of life, this champion aeronaut flies more than 2.4 million kilometres - the equivalent of three return journeys to the Moon.

The Arctic tern's marathon flight is fairly familiar, but estimating the length of such a massive trek isn't easy. It would be charitable to forgive scientists for getting it wrong, given that they had to rely on observations at sea and capturing banded birds at different places. But few would have predicted just how wrong the textbook figures are. They typically suggest that the tern covers 40,000km in a year. The bird should be insulted - in reality, it flies almost twice that amount.

Its true itinerary has only just been revealed through the use of tiny tracking devices. Similar machines have already exposed the travel plans of larger seabirds like albatrosses, petrels and shearwaters. But these gadgets been too large and clunky to attach to smaller fliers - strapping a 400g recorder to a 100g bird isn't going to give you an accurate picture of its flying abilities.

Carsten Egevang from Denmark's Aarhus University changed all of that by developing tiny geolocators, less than 1g in weight. These locators can track the movements of migrating birds by recording the amount of light falling upon it at different points in its journey, and they've already been baptised by recording the entire migration of songbirds. Egevang strapped them to the leg of 50 terns, and managed to retrieve 11 of them the following season, when the birds returned.

The southbound migration is the more arduous leg of the journey. At the end of the breeding season, the terns take off from Greenland and Iceland and head southwest to a stopover that we didn't know about. It sits in the middle of the North Atlantic,where the rich, food-filled waters of the north meet the warmer but less productive southern swirls. The terns spend an average of 3-4 weeks there between August and September, galvanising themselves before heading southeast to Africa.

They all follow the same path until they reach the Cape Verde Islands, off Africa's western bulge, where they split into two groups. One faction continues to hug the African coast, while the other crosses the Atlantic and follows the curve of Brazil. At about 40 degrees South, both groups shift from a southerly flight to messier east-west movements, some even making it as far as the Indian Ocean.

By November, they've all reached their destination. On average, it has taken them 93 days, although the fastest fliers make it in just 69. And they're rewarded for their trouble. Perpetual sunlight starts to bathes the Antarctic coast, giving them round-the-clock opportunities to plunder the rich local seas for krill and other food. They stay there for several months but come early-mid April, it's time to head north again.

This journey is more direct. Full of krill-power and aided by favourable winds, the terns cover a massive 500km every day and it takes them just 40 days on average to get home. They eschew their coast-hugging routes in favour of flying straight over deep water. They carve an S-shape through the atmosphere, heading to the southwest corner of Africa, crossing the Atlantic, and turning back to the same North Atlantic transit point they used in their southbound leg. They finally arrive home in the Arctic in May, exhausted and ready to breed.

The Arctic tern is certainly the most accomplished of feathered migrants, but it's far from the only one. Many birds nest in the high Arctic only to travel further south in the winter. But migration isn't a walk in the park - it's an uber-marathon that soaks up energy and exposes birds to extreme weather. Many die en route and even the survivors must cope with the extreme Arctic environment, and be strong enough to breed in it.

Why undergo such epic travels? There must be truly massive benefits of a more northerly final destination to outweigh such significant costs. Previous studies have found two answers - higher latitudes mean fewer parasites, and the longer daylight hours give the birds more time to catch they food they need. But birds could still enjoy these benefits by stopping further south in the sub-Arctic, cutting their journeys significantly and breeding in more forgiving climates.

Now, Laura MicKinnonfrom the University of Quebec has a third answer that might explain the final northwards push - it's safer. McKinnon studied the influence of predators in a continent-wide study. She set up over 1,500 artificial nests at breeding sites throughout Canada, from the sub-Arctic latitudes of 53 degrees, to the High-Arctic at 83 degrees.

Studying predation is a messy business. Parent birds could compensate for the threat of hungry jaws by mounting vigorous defences, relying on camouflage, or breeding at opportune times. These sorts of variables would usually flummox an experiment like this, but McKinnon's artificial tests allowed her to strip them all away and focus on location, location, location.

She found that breeding one degree of latitude higher reduces the odds of being eaten. In the experiment, 29 degrees separated the northernmost and southernmost breeding sites - that translates to a 65% lower risk of falling to predators. That's a massive boon by any bird's standards, but can it truly compensate for the energetic cost of migration? That's a question for another study.

References: Egevang et al. 2010. Tracking of Arctic terns Sterna paradisaea reveals longest animal migration. PNAS http://dx.doi/10.1073/pnas.0909493107

Mckinnon et al. 2010. Lower Predation Risk for Migratory Birds at High Latitudes. Science

Image by Oddurben

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