The bird that chases daylight

The Arctic tern is a small seabird: it weighs less than 125 grams. And every year it makes what researchers describe as the longest seasonal journey of any animal.

Measuring the impossible

For decades, scientists knew roughly where Arctic terns went. Ringing records and sightings at sea showed that the birds breed in the far north and spend the northern winter in the Southern Ocean, near Antarctica. What nobody could do was follow a single bird the whole way.

That changed when researchers fitted eleven Arctic terns with geolocators weighing just 1.4 grams. The results, published in the scientific journal PNAS in 2010, showed the birds travelling much further than expected. Some individuals covered more than 80,000 kilometres in a year.

The average across the tracked birds was about 70,900 kilometres — close to twice the figure that had often been quoted before, according to a summary by the Encyclopedia of Puget Sound.

Not a straight line

The routes were as interesting as the distances. On the way south, birds from the same colony split: some followed the coast of Africa, others the coast of South America. On the way back north, all of them followed a broadly similar S-shaped path through the Atlantic.

That apparent detour makes sense. The researchers found that the terns targeted areas of the ocean rich in food, both as stopovers and as wintering areas, and used the planet’s prevailing wind systems to reduce the cost of flying.

A life in the light

Because the Arctic tern spends the northern summer in the north and the southern summer in the south, it lives through long days at both ends of the planet. The team behind the tracking dataset suggests this means the Arctic tern probably experiences more daylight in a year than any other creature on Earth.

Arctic terns can live for more than 30 years. Added up over a lifetime, the distance one bird flies may exceed 2.4 million kilometres — roughly equal to three return journeys to the Moon.

Why it matters

A journey like this depends on many places staying healthy: breeding colonies in the north, rich feeding grounds in the open ocean and food-filled waters in the far south. When a single small bird links the Arctic to Antarctica, protecting nature becomes a shared, worldwide task.