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.
We know now that birds undertake incredible odysseys across the planet to avoid cold winters, and to find abundant food and shelter.
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.
How are migratory animals threatened?
Roughly half of species listed in the Convention on the Conservation of Migratory Species of Wild Animals (CMS) are showing population declines, and 22% are threatened with extinction.
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.
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.
Climate change is also disrupting weather patterns that let animals know when to start their migrations.
How can we protect wild animals from extinction?
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.
