A single bird flies from the Arctic to Antarctica and back every year

Most migratory birds pick one hemisphere’s summer and stick with it, flying just far enough to avoid winter. The Arctic tern does something far more extreme: it breeds under the near-constant daylight of the Arctic summer, then flies to the opposite end of the planet to spend the northern winter in the equally bright summer of Antarctica. The result, confirmed only in the last decade once tracking devices became small enough to attach to a bird this size, is the longest migration recorded for any animal on Earth. A tern that completes this same round trip every year for three decades effectively spends more of its life in daylight than any other creature alive.

A Tern Tracked for Nearly 60,000 Miles

Researchers at Newcastle University fitted Arctic terns nesting on England’s Farne Islands with tiny geolocators, devices weighing well under a gram that record light levels every few minutes and let scientists reconstruct a bird’s approximate position from patterns of sunrise and sunset. One tracked bird, believed to be female based on her wingspan and body proportions, flew nearly 60,000 miles over a single annual round trip, departing the Farnes in July, reaching the tip of South Africa within a month, wintering in the Indian Ocean, then continuing on to the edge of Antarctica before looping back north along the coast of West Africa the following spring.

A Zigzag Route Built Around Wind, Not Distance

The tern’s path is anything but a straight line between the poles. Researchers who analyzed the tracking data found that the birds make detours of a thousand miles or more, riding global wind patterns rather than cutting directly across oceans, which paradoxically saves energy even though it adds distance. Because geolocators only estimate a bird’s position once or twice a day from light readings, scientists say the nearly 60,000-mile figure is almost certainly an undercount of how far the birds actually fly, since it misses smaller movements between those fixed points. Researchers involved in the tracking project have said that a GPS-based device, once engineering catches up enough to make one light enough for a bird this size to carry, would likely reveal an even longer true distance.

A Small Bird With an Outsized Frame for the Job

Arctic terns are not built like typical long-haul specialists. An adult, formally Sterna paradisaea, measures only about 13 to 14 inches from bill to tail, weighs between 3 and 4.5 ounces, and carries a wingspan of roughly 30 to 33 inches, slight proportions built for riding wind rather than for carrying large fat reserves. A follow-up analysis of the Farne Islands tracking data, published by Newcastle University in 2019, filled in more of the route the original geolocators had only sketched: it found the birds sometimes make an 8,000-kilometer, 24-day nonstop crossing of the Indian Ocean while feeding on the move, and on other legs take an overland detour across Ireland before dropping in for a stopover at Llangorse Lake in Wales’s Brecon Beacons National Park on the return journey north.

A Lifetime Spent Chasing Two Summers

An Arctic tern that lives for around 30 years and repeats a migration of this scale annually could log more than 1.5 million miles in its lifetime, comparable to multiple round trips to the Moon. The birds rely on the Farne Islands and similar colonies purely for breeding, since suitable nesting habitat and abundant sand eels make the islands useful for only part of the year, then depend on the Antarctic’s summer feeding grounds for the food that fuels the return trip. That dependence on two distant, specific environments, thousands of miles apart, is part of what makes Arctic terns unusually vulnerable to change in either hemisphere, even though the global population is not currently considered threatened.

Feeding by Plunge-Diving, Resting Almost Never

Arctic terns catch fish by plunge-diving, climbing roughly 30 meters above the water and dropping directly onto prey near the surface, a technique that leaves little room for the slow soaring or gliding that helps many long-distance migrants conserve energy. Tracking data has shown the birds moving almost continuously, day and night, throughout their migration, and researchers studying the Farne Islands colony over nearly two decades say they still do not fully understand how the birds sustain that level of activity across tens of thousands of miles. Because the terns are so sensitive to disruptions in food availability, particularly the sand eels that both they and puffins depend on, scientists have described them as an early warning system for changes moving through the wider marine food web.

A Record That May Not Stand Forever

Researchers tracking the Farne Islands terns have also noted a soft decline in numbers there, worsened by a run of severe summer storms, with related colonies in Iceland and Scotland faring even worse as they are forced to travel farther for food. The Arctic tern’s closest rival for extreme flight is the bar-tailed godwit, a shorebird that holds the record for the longest nonstop flight without landing to rest or feed, but no animal yet tracked matches the tern’s combined annual distance across an entire year. That combination of scale and endurance is why, even after multiple record-breaking tracking studies, researchers who study the species still describe following its full migratory path as one of the more remarkable ongoing puzzles in bird conservation.

This article was created with the assistance of AI and reviewed by an editor.

More from Morning Overview