Every September, a group of small birds weighing less than 100 grams sets off from the Arctic on a journey across the globe. They fly southward, crossing the Atlantic and Indian Oceans, and finally reach Antarctica—these are Arctic terns, the species with the longest migration distance in the animal kingdom, traveling 70,000-95,000 kilometers each year.

The Arctic tern’s migration route is a wonder of nature. They breed in the Arctic summer, and when the Arctic enters polar night, they fly to Antarctica to enjoy the summer sun, living in daylight throughout the year. Scientists using satellite tracking found that an Arctic tern codenamed “E7” completed the migration from Alaska to Australia in 115 days, with a total distance of 135,000 kilometers. The oldest Arctic terns live for more than 34 years, and the total migration distance in their lifetime is equivalent to traveling to the moon and back four times.

To complete this epic journey, Arctic terns have evolved a perfect flight structure. Their wings are long, thin, and pointed, with an extremely high wingspan-to-weight ratio, enabling them to efficiently use air currents for gliding and reduce energy consumption. During flight, they can sleep while flying—their brain hemispheres rest alternately, always keeping one eye open to observe the environment. This “half-brain sleep” pattern allows them to fly continuously for weeks without rest.

The Arctic tern’s migration is not only to find food but also to avoid natural enemies and take advantage of the best breeding conditions. When they breed in the Arctic, they can avoid threats from most birds of prey; when they overwinter in Antarctica, they can enjoy the season when krill resources are abundant. This precise migration timing is an instinct formed through tens of millions of years of evolution, and an outstanding example of life adapting to changes in the Earth’s environment.