The Arctic tern is a small seabird you could almost hold in one hand: a black cap over a pale body, a forked tail, and a wingspan under a metre. Every year it flies from the top of the planet to the bottom and back. And because it follows summer at both ends of the Earth, it very likely sees more daylight than any other animal alive.
The scale of the journey
The numbers stop you first. In a 2010 study led by Carsten Egevang and published in the Proceedings of the National Academy of Sciences, researchers fitted 11 terns from Greenland and Iceland with tiny tags weighing 1.4 grams that record light levels. The birds made an average round trip of about 70,900 kilometres a year. That is far longer than the straight-line distance between the poles, because the birds do not fly straight.
The shortest direct distance between the Arctic breeding grounds and the Antarctic is roughly 19,000 kilometres. The terns cover several times that, following S-shaped routes down the Atlantic to ride the winds rather than fight them. One bird from the Farne Islands, tracked over ten months, logged about 96,000 kilometres (60,000 miles), one of the longest annual migrations ever recorded for the species.
What makes this harder to picture is how small the bird is. An Arctic tern weighs little over 100 grams, less than a small apple. A body that light crosses the whole planet twice a year.
Two summers, and what that means
The tern breeds in the Arctic during the northern summer, when the far north tips toward the sun and daylight lasts almost 24 hours near midsummer. Then it flies south and reaches Antarctic waters for the southern summer, when the bottom of the planet is the part leaning into the light.
So the bird is in the far north during its long summer day, then in the far south during the opposite hemisphere’s long summer day. It spends the dim in-between seasons flying over open ocean, not sitting in the dark. It is always following the summer, and that is the whole point of the migration.
The trip home is much faster than the trip out. According to Cool Antarctica, the spring journey north takes about 40 days, against about three months heading south, because the birds pick wind-assisted routes rather than the most direct ones.
More daylight than any other creature
Chase summer at both ends of the Earth and the daylight adds up in a way it cannot for an animal that stays in one place.
In the summary of his tracking work, Egevang writes that the Arctic tern “probably experiences more sun light during a calendar year than any other creature on Earth.” National Geographic’s education resource puts it similarly, writing that the tern “may experience more daylight than any other animal.” Cool Antarctica reaches the same conclusion.
No one has measured every animal’s yearly sunlight, so “probably” and “may” are doing honest work. The logic, though, is hard to argue with. An animal that spends both summers near the poles, where the sun barely sets, and skips most of both winters by being somewhere else, is the obvious candidate.
What the migration costs, and what it buys
A journey on this scale is not free, and the striking thing is how long the birds keep making it. Arctic terns are long-lived for birds their size. The Cornell Lab of Ornithology notes that they can live for decades, and the oldest recorded bird was at least 34 years old.
Stack those round trips together and the lifetime total becomes almost absurd. Egevang’s summary puts it at around 2.4 million kilometres, roughly three return trips to the Moon, flown by a bird weighing little more than 100 grams.
The reason for all this becomes clear once you see it. Summer at high latitudes is when the ocean has the most food and the days are longest for feeding. By moving between hemispheres, the tern arranges its whole year so it is almost always somewhere rich and bright, rarely stuck in the lean, dark part of a season. The enormous flight is the price of largely avoiding winter.
Most animals adapt to the seasons where they live. The tern’s answer is to refuse to stay anywhere long enough for winter to settle in, paying for that refusal in kilometres.
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