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The common carrion It is a medium-sized bird that is mostly pale in color, with a gray back and wings and a whitish tail. On his head he wears a highly visible black hood that varies in color depending on the time of year.
One of its closest relatives is the cartán ártico (Sterna paradisaea) a bird that stars in one of the most extraordinary stories of the animal kingdom. Unlike the common tern, during the breeding season its beak is completely red, as are its legs, which take on a dark greyish color outside of that period of the year.
This species is monogamous, it reproduces in the arctic and subarctic regions of North America, Asia and Europe, with the largest concentration of colonies in the world being located in Greenland.
They nest at ground level and their annual laying ranges between three and five eggs.
A journey of epic dimensions
Once the reproduction has taken place, at the end of August, they head towards the Southern Ocean. In this way, weighing just 125 grams, the arctic tern carries out one of the longest seasonal migrations in the entire animal kingdom, being able to cover more than 80,000 kilometers per year.
It is a journey that is not exempt from dangers, which exposes these birds to extreme weather conditions and high energy expenditure, so many of them will perish before reaching their destination.
It is estimated that this bird can live regarding three decades, so, following a little mathematical calculation, throughout its life it will have traveled around 2.4 million kilometersa figure equivalent to six times the distance between the Earth and the Moon.
Thanks to tiny tracking devices, scientists have observed that during the trip they make a significant stop, which can sometimes last up to a month, in an area north of the Azores islands. There they replenish their strength, feeding on fish and small marine crustaceans that they capture with dives and dives.
Researchers have also discovered that the arctic tern is capable of detecting ocean currents, which is essential for its feeding, since with them nutrients are raised that attract plankton, which in turn is the claim of small fish that , finally, they will end up being part of the feeding of the terns.
The return trip is much faster
Then they continue their course towards the south and at the height of the islands of Cape Verde flocks of terns split into two populations, one traveling by Africa and another that does it for South Americaalong the coast of Brazil. Both groups will arrive in Antarctica in December, around four months following leaving their camps Greenland.
In the destination, life is not easy either, they have to overcome many dangers, from natural predators such as the mink, to the loss of habitat or key prey, as a result of climate change. In any case, the risk is worth it, since they take advantage of their stay on the Antarctic coast to dive in search of krill and other food for long days.
They will remain there until mid-April of the following year, at which time they begin their return journey. This will be much faster, thanks to the fact that they take advantage of the invaluable help that the winds offer them. In this way, in just two months they are back in their home in the north, they arrive exhausted but ready to mate.
With its return, the arctic tern closes its annual cycle, a journey that joins the two poles, one where it breeds and the other where it spends the rest of its non-breeding time.
Pedro Gargantilla is an internist at El Escorial Hospital (Madrid) and the author of several popular books.
