Animal migrations are one of nature's great marvels, but nomadic creatures are under increasing threat. Why do animals migrate, and how can we protect them?
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For centuries, the disappearance of birds during winter was a great mystery to many Europeans. Some thought they hibernated underwater, or that they transformed into other species, or even went to the moon.
But by the 1800s, evidence of remarkable animal migrations started to emerge. Famously, German hunters in 1822 shot down a white stork that had a 75-centimeter (30-inch) African wooden spear lodged in its neck — proof that it had traveled between the continents. Its preserved remains are still on display in the University of Rostock, Germany.
Every year, birds such as the white stork fly more than 10,000 kilometers (6,200 miles) south for the winter, only to make the return journey in time to catch spring. Some birds, such as the bar-headed goose, fly high over the Himalayas on their migrations, while Arctic terns travel pole to pole in search of an endless summer.
It's not just birds. Species of fish, mammals, insects and reptiles all undertake long-distance expeditions.
The European eel can travel about 10,000 km over two years to reach its breeding grounds near the Bahamas. Gray whales summer in the cold northern Pacific before traveling to the coasts of California and Mexico. And herds of over a million wildebeest migrate hundreds of kilometers during the dry season in search of water in the Serengeti of Kenya and Tanzania.
Practically all CMS-listed fish are threatened with extinction.
Because migratory species travel such long distances, they must negotiate human-made barriers and dangers on their long journeys.
For birds, the perils include hunting, light pollution and large buildings. Hunters in the Mediterranean kill tens of millions of migratory birds each year, and as many as a billion die each year after flying into buildings.
Aquatic animals are also endangered by pollution and fishing, while land animals face physical barriers such as roads and fences. Some wildebeest migration routes have completely collapsed due to farm fences.
Migratory birds: The real long-distance champions
For any human flier, 14,000 kilometers in one stretch would be a challenge. But as birds cross oceans and continents, many species — including endangered ones — pull off this feat without the aid of jet engines.
Image: AP Photo/David Guttenfelder on assignment for National Geographic Magazine
The long-distance champion
Meet the bar-tailed godwit: Breeding primarily on Arctic coasts, and the Scandinavian and Siberian tundra, this wading species spends the winter in Australia and New Zealand. In 2007, a tagged bird was recorded as having flown 11,600 kilometers (7200 miles), from Western Alaska to New Zealand, in a single nine-day stretch. That probably makes it the record-holder for non-stop flight.
Image: picture-alliance/dpa/McPHOTO
The little guy
Named for the bright red plumage under its beak, the ruby-throated hummingbird can grow up to 9 centimeters (3.5 inches) and weigh as little as 3 grams. But don't let the bird's small stature fool you, it still gets around. During migration, some specimens fly 900 miles non-stop across the Gulf of Mexico to their breeding grounds in the Eastern United States.
Image: Imago/D. Delimont
The officer
Belonging to the stork species, the greater adjutant owes its name to its gait — which is said to resemble that of a military officer. But the adjutant doesn't command much land anymore. It's listed as endangered, and is only found in two breeding colonies in India and Cambodia. Outside of the breeding season, it visits neighboring countries across Southeast Asia.
Image: picture-alliance/dpa
'The Bird'
An invasion of sooty shearwater birds in California in 1961 was the inspiration for Hitchcock's famous thriller "The Birds," which came out two years later. In real life, the birds are more awe-inspiring than scary. Crossing the Pacific and Atlantic, up coasts in spring and down in autumn, they clock up to 14,000-kilometer journeys. Oh, and they can also dive down to more than 60 meters.
Image: picture-alliance/dpa/K. Wothe
The sewing machine
Dunlins are small waders, and breed in Arctic regions. While those from Northern Europe and Asia fly as far as Africa for the winter break, their Alaskan and Canadian counterparts prefer the much closer North American coasts. Their characteristic mechanical way of picking at food has earned them their nickname.
Image: picture-alliance/dpa/M. Woike
The snowbird
Arctic terns have developed the ultimate strategy to evade winter: They breed in the Arctic during the northern summer, then travel to the shores of Antarctica, racking up 80,000 kilometers of travel or more per year in the process. That means they see two summers. But winters? Zero. Every year.
Image: picture-alliance/dpa/H. Bäsemann
The holy bird
Critically endangered, the northern bald ibis is now only found in southern Morocco. It used to migrate across Europe, North Africa and the Middle East. Revered by the ancient Egyptians and said to have been the first bird to be released from Noah's Ark, Turkish pilgrims also looked to them for guidance on their way to Mecca. Spiritual appreciation hasn't helped stem its decline.
Image: picture-alliance/dpa/M.Unsöld
The all-rounder
The northern wheatear has nothing to do with wheat or ears but it does make one of the longest migrations of any small bird. It passes over oceans, snow and desert during its spring journey from Sub-Saharan Africa to the Northern Hemisphere. There, the birds occupy an area stretching from Central Asia to Europe and Alaska. Trackers have shown they can travel 290 kilometers a day on average.
Image: picture-alliance/blickwinkel/M. Woike
The commoner
The common crane is a common appearance in the north of Europe and Asia, where it breeds typically in moors and wetlands. For its winter vacation, it commonly migrates to North and East Africa, as well as to Israel and parts of Iran.
Image: picture-alliance/dpa/J. Büttner
The harlequin
Anything but boring, the ruddy turnstone's plumage features a harlequin-like pattern during summer when it breeds in the Arctic Tundra of Europe, Asia and North America. As temperatures drop, the small wader changes into its brown winter coat and migrates south on coastlines around the world.
Image: picture alliance/blickwinkel/M. Woike
End of a journey
These ducks had made it across the Mediterranean from North Africa only to be shot on arrival at this beach in Albania. Every year, hunters kill many millions of migratory birds across the Mediterranean — for food or money, or, most controversially, sport.
Image: AP Photo/David Guttenfelder on assignment for National Geographic Magazine
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Two of the biggest threats to migratory species are overexploitation — including unsustainable hunting, overfishing and bycatch — and habitat loss caused by humans expanding farms, cities and other infrastructure.
While traveling, animals must refuel and rest. But fragmented landscapes and degraded habitats on just one stop along their journeys can cause them to starve.
The CMS, or Bonn Convention, signed in 1979, helps countries coordinate efforts to protect migratory species along their routes. Parties meet every two to three years to monitor progress and formulate new plans.
Thanks to the convention, animals such as Central Asia's Saiga antelope face less habitat loss and poaching. Several species of migratory shark can also find safe havens in marine protected areas, and albatrosses are killed much less often by fishing trawlers.
But climate change and habitat loss continue to threaten migratory species. Conservation groups say more needs to be done to protect this important group of animals, including conserving and restoring habitats and removing barriers along migratory corridors.
These great migrators perform vital roles in the ecosystem such as pollinating plants and spreading seeds, thus forming part of the food web. They also have enormous cultural significance, inspiring art, religion and literature.