Every year, billions of migratory animals are on the move on our planet. They traverse the oceans, fly over the world's highest mountain ranges, roam deserts and cross borders, following the same millennia-old routes year after year. Migratory animals are global commuters. They face hazards on their journeys, many undergo incredible metamorphoses, some are as punctual and reliable as Swiss watches, and others cover enormous distances in the course of their lives. For thousands of years, their journeys have been in sync with the seasons and the photoperiod, the length of the day, and since time immemorial their migration has fascinated humans and given rise to myths and legends. Today, however, migratory animals face new kinds of threats: noise and light pollution, insecticides, roads and fences, and of course climate change.

Some migratory animals are small and feathery, like butterflies, weighing less than a gram. Others are huge and weigh up to 150 tonnes, like the blue whales. Birds, mammals, amphibians, insects and fish migrate: from tuna to sardines, from salmon to eels. Even animals you wouldn't expect: crabs and reptiles like sea turtles. The loggerhead turtles that are born in Florida undertake a migration of over 16,000 kilometres in the Atlantic Ocean immediately after birth. Many of them reach the Mediterranean Sea. As a result, 45 percent of the young specimens there come from America.

Migratory animals have some common characteristics. They have excellent orientation: they follow the sun, the stars, the earth's magnetic field and often their routes are “inscribed” in their genes. Most of them live in places with precisely defined seasons and very often it is precisely their change that means that the areas that are advantageous and rich in food in winter are not suitable for reproduction in summer - and vice versa. This is why loyalty to a particular location means death. Many migratory animals have to move to areas that are completely different from the ones they live in in order to reproduce: Sea turtles, for example, have to return to the beach to lay their eggs after spending years in the ocean. Migratory animals are forced to move constantly. They expose themselves to countless dangers in the process, accepting probable death to escape certain starvation.

“Until a century ago, we knew nothing about these journeys. We knew neither routes, nor timings, nor orientation systems”

It is estimated that there are fifty billion migratory birds in the world. About five billion of them migrate every spring from Africa, where they spend the northern winter, to Europe and Asia. Swallows, bee-eaters, storks, warblers: they seek out these regions to breed and take advantage of two great benefits. Firstly, in our latitudes in spring there is an explosion of flowers, fruits and insects, the staple diet of many birds. Secondly, the days become longer: there is more daylight available for hunting and rearing offspring. If they stayed in Africa, they would not have this abundance. When summer ends and winter approaches, migratory birds return to Africa, often to areas south of the Sahara, where they find a new “spring”.

Until a century ago, we knew nothing about these journeys. We knew neither routes, nor timings, nor orientation systems. The fact that we have found a scientific answer to the question “Where do migratory birds go on their journeys?” is thanks to the Dane Hans Christian Mortensen, headmaster of the grammar school in the city of Viborg and an avid birdwatcher. In order to find out where the birds he sees in his garden in spring disappear to in winter, Mortensen invents a method in 1890: bird ringing. For the first time in the history of ornithology, Mortensen captures starlings and attaches a small zinc ring to their legs with the inscription “Viborg 1890”. Then he releases them, hoping to get news about the birds soon. And indeed: years later, his ringed starlings are found again in Norway and in the Netherlands and identified thanks to that ring on the leg. Almost everything we know about migratory birds today is thanks to bird ringing.

Today, migratory birds and other migratory animals are also tracked by radar, GPS and data loggers: tiny devices that are able to register position data. Thanks to the latter, for example, the record migration of the Arctic tern has been tracked: a bird weighing just a hundred grams that nests along the Arctic Circle and spends the season corresponding to our winter in Antarctica, covering 90,000 kilometres in flight every year. This is the longest migration in the entire animal kingdom. On average, the Arctic tern covers 2.5 million kilometres during its lifetime, a distance six times greater than the distance between the Earth and the Moon. In addition, thanks to GPS technology, we have brought to light another “impossible” animal endeavour: the Amur falcon, a bird of prey weighing about 150 grams, reproduces in East Asia, more precisely in Manchuria, on the other side of the Amur River, after which it is named. But it spends the winter in southern Africa, after a long flight over the Indian Ocean. The Amur falcon always sets off on its journey south at the end of August. It crosses China, flies along the Himalayas, reaches north-east India and Bangladesh. Then it tackles the last stage: the flight over the Indian Ocean, taking advantage of the monsoon winds. A 3,500-kilometre route to reach southern Africa via the Maldives and the Seychelles, making the crossing alongside another animal: the migratory dragonfly, its preferred snack.

“The economies of many countries depend on migratory animals, fishing and tourism”

Today, however, the journey of many migratory animals is in danger of being interrupted forever. Plastic waste in the oceans is a major problem for sea turtles and whales, which swallow it; light pollution may be responsible for birds losing their bearings or freshly born baby turtles taking the wrong path so that, after hatching from the sand, they do not head for the sea but are attracted by the lights of beach bars or coastal roads. Roads and fences also disrupt the routes of mule deer and pronghorn in Wyoming and of common toads and green toads in Europe. Even the routes of one and a half million blue wildebeest that migrate between Kenya and Tanzania were threatened by a planned highway.

The biggest problem, however, is climate change. Reindeer and caribou and other large migratory animals are literally threatened with starvation because of climate change: the global population of these species has declined by more than half in the past twenty years. For an unfortunately almost banal reason: the lichens they get by scraping away the snow with their hooves are the only source of food available in winter. In the Arctic, however, it rains more and more often and the rain freezes on the ground. As a result, the reindeer and caribou can no longer reach the lichens. Meanwhile, migratory birds no longer make it to Europe in time for spring: In our latitudes, spring sets in earlier every year, as does the swarming of insects. And no matter how much the birds try to gain time by reducing the duration of breaks during the journey, that perfect synchrony between the arrival of migratory birds at the breeding grounds, the hatching of the chicks and the peak of food abundance, which has developed over thousands of years, is gradually being lost. Climate change is also already altering ocean currents and the feeding stations vital to many marine migratory animals, not to mention ice melt in Antarctica, which is also affecting two other migratory species: emperor penguins and Adelie penguins.

If we lose the migratory species, we don't just lose biodiversity. We also lose many of the benefits we derive from our environment: Migratory birds rid us of plant pests and other insects that are harmful to humans, like mosquitoes. A swallow, for example, can eat thousands of mosquitoes a day. The economies of many countries also depend on migratory animals: not only fishing - think of bluefin tuna, sardines, Icelandic capelin, salmon - but also the tourism sector: from birdwatching to whale watching to the egg-laying and hatching of the various species of sea turtles. These are all events that attract significant numbers of tourists.  And finally, without migratory animals, we would also lose some landscapes: from the rainforest of Christmas Island, worked by red land crabs, to the savannah between Kenya and Tanzania, fertilised by wildebeest, to the tundra “managed” by reindeer and caribou. So if we lose the migratory animals, we lose not only gross domestic products, but in the end we will also say goodbye to the beauty of our world.