How did an arrow stuck in a stork show that birds fly to Africa?

It came back with the proof inside it. In 1822, a white stork was found in Mecklenburg, in Germany, with an arrow made from central African hardwood still stuck in it. The bird, nicknamed a Pfeilstorch, or "arrow stork," had evidently been shot in Africa and then flown back to Europe, and it became some of the earliest evidence of long-distance stork migration, rather than birds vanishing some other way each winter.

That question was ancient. Aristotle recorded that cranes travel from the steppes of Scythia to marshes at the headwaters of the Nile, but he also suggested that swallows hibernated. The idea stuck around remarkably long: even the naturalist Gilbert White, in 1789, repeated a story of swallows found in a collapsed chalk cliff, and as late as 1878 one writer could list 182 papers on the hibernation of swallows. Only at the end of the 18th century was migration accepted as the explanation for birds disappearing from the north in winter.

We now know migration is a seasonal round-trip between breeding and wintering grounds, made by roughly 1,800 of the world's 10,000 bird species, usually timed by the changing length of daylight. Food appears to be the main driver: some hummingbirds skip migration entirely if people keep feeding them through the winter, and the long days of a northern summer simply give parents more hours to feed their chicks. The record-holder for sheer distance is the Arctic tern, which travels between Arctic breeding grounds and the Antarctic every year, while a Manx shearwater can cover 14,000 kilometres between its nesting burrow and the Southern Ocean.

Finding the way across such distances relies on several senses working together: a Sun compass that adjusts for the time of day, visual landmarks, smell, and a genuine sense for Earth's magnetic field. On a bird's very first migration, it seems to fly in the right direction purely by sensing that magnetic field through light-sensitive chemical reactions in its eyes, a system that works only during the day and has nothing to do with where the Sun actually is. With experience, it adds a second layer: magnetite in its trigeminal system that registers the field's strength, which changes with latitude, letting an older bird map landmarks and know when it has reached its destination.