Arctic terns fly from pole to pole, the longest migration of any animal on Earth

A small seabird weighing barely as much as a smartphone spends its life chasing daylight from one end of the planet to the other. The Arctic tern breeds in the Arctic and subarctic during the northern summer, then flies to the edge of Antarctic sea ice for the southern summer, logging a round trip that dwarfs the travels of any other animal known to science. Researchers who tracked the birds with tiny geolocators found some individuals covering more distance in a single year than most people travel in a lifetime of commercial flights.

A circumpolar breeder with a six-month commute

The Arctic tern, Sterna paradisaea, nests across the Arctic and subarctic zones of Europe, Asia, and North America, with colonies stretching as far south as Brittany in France and Massachusetts in the United States. Unlike birds that migrate a few hundred or even a few thousand miles, the Arctic tern treats the entire globe as its range. It breeds during the brief northern summer, then abandons the region entirely as autumn approaches, tracing a path down the length of the Atlantic, Pacific, or Indian Ocean basins depending on where a given population starts. The species was formally described in the 18th century, but only in the last two decades have biologists been able to measure exactly how far individual birds travel, thanks to lightweight tracking tags that record ambient light levels to estimate location.

The numbers behind the longest migration on record

Tracking studies summarized by the Arctic tern’s reference entry show that birds nesting in Iceland and Greenland average roughly 70,900 kilometers (about 38,300 nautical miles) on their annual round trip between hemispheres. Birds breeding farther south, in the Netherlands, cover a shorter but still enormous average of about 48,700 kilometers. The most extreme measurement on record belongs to a single tern tagged in the Farne Islands off Northumberland, England, which covered roughly 96,000 kilometers in just ten months of tracked movement. That is comparable to circling the Earth at the equator more than twice in under a year, achieved by a bird that weighs around 100 grams and measures only 33 to 36 centimeters from bill to tail.

Why the route zigzags instead of running in a straight line

An Arctic tern does not fly the shortest possible line between its Arctic breeding ground and Antarctic wintering waters. Instead, the birds trace a winding path that takes advantage of prevailing wind systems, which reduces the energy cost of a journey that would otherwise be punishing for an animal this small. On the southbound leg, many terns swing toward the coast of West Africa before angling across the mid-Atlantic toward South America and then continuing south to the edge of Antarctic sea ice. The return trip north often follows a different track, running up the center of the Atlantic rather than retracing the outbound route. Along the way, the birds make extended stops to forage, particularly in productive waters near the Antarctic ice edge, where food is abundant during the southern summer.

Chasing perpetual summer and near-constant daylight

The reason for this extraordinary effort comes down to light and food. By breeding in the Arctic during its short, intense summer and then relocating to the Antarctic for its summer six months later, the Arctic tern effectively lives in two summers every year and almost never experiences a true winter. Because both polar regions see extended daylight during their respective summers, the Arctic tern is thought to experience more hours of daylight over the course of a calendar year than any other creature on the planet. That extra light translates into more time to forage, since the species feeds mainly on small fish and marine invertebrates that it catches by plunge-diving into open water. The strategy trades a grueling annual commute for near-continuous access to feeding conditions that would otherwise be available only part of the year in a single hemisphere.

A lifetime of flying equivalent to trips to the Moon

Arctic terns are also comparatively long-lived for a bird their size, with individuals commonly reaching 20 to 30 years of age and some documented well beyond that span. When the annual round-trip distance is multiplied across a three-decade lifespan, the cumulative total reaches approximately 2.4 million kilometers, a figure that researchers have compared to roughly three round trips between the Earth and the Moon. Few animals of any size approach that lifetime travel figure, and no other species is known to combine that distance with the pole-to-pole route that defines the Arctic tern’s annual cycle. The comparison underscores just how much of the bird’s life is spent in transit rather than at either of its two seasonal homes.

A conservation status that still depends on distant habitats

Despite the extremity of its migration, the Arctic tern currently holds a conservation status of Least Concern under the International Union for Conservation of Nature’s Red List criteria, reflecting a large and widely distributed global population. That status does not mean the species is free of risk. Because Arctic terns depend on breeding colonies scattered across the Northern Hemisphere and on foraging conditions near Antarctic sea ice thousands of kilometers away, changes in either polar region, from shifting sea ice patterns to disturbance at nesting sites, can affect the birds at both ends of their annual journey. The reliance on two distant and ecologically distinct habitats makes the Arctic tern a useful indicator species for researchers studying how polar ecosystems are changing, since a disruption at either pole has the potential to ripple through a migration route that spans nearly the entire planet.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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