Ed Yong in The Atlantic:
In 1250, the prior of a Cistercian Abbey reputedly tied a note to a leg of a barn swallow, which read: “Oh swallow, where do you live in winter?” The next spring, he got a response: “In Asia, in the home of Petrus.”
This perhaps apocryphal story marks one of the first known instances of someone tagging an animal to track its movements. Thanks to many such endeavors, we now know that every year, barn swallows migrate between their breeding grounds in the northern hemisphere to wintering grounds throughout the tropics and the south. In 1912, one intrepid individual that was ringed in England turned up 7,500 miles away in South Africa.
But swallows are the exception rather than the rule. The journeys of most migratory animals, especially smaller species, are a mystery. Flocks, herds, and shoals are constantly crisscrossing the globe, but despite the intense surveillance of our planet, we often have no idea what paths they take. “They leave in one place and we don’t know what happens to them until they show up in another place,” says Meg Crofoot from the University of California, Davis.
This ignorance makes it hard to save threatened species: what works in one part of the world may be completely undone as animals travel to another. It also jeopardizes our own health. Where are the birds that harbor avian flu? Where do the bats that carry Ebola go? What about the red-billed quelea, a small finch that flocks in millions and devours crops with locust-like voraciousness?