For centuries, people in Europe didn't really know where birds went during the winter. It's not their fault, they had a lot on that kept them from investigating the disappearance of other species. It's hard to focus on "where did birb go?" when you're working on your main task of dying of the plague.
The main theory, which went all the way back to Aristotle and ancient Greece, was that birds hibernated during the winter and that summer redstarts turned themselves into winter robins for the colder months, while garden warblers turn into blackcap warblers. As outlandish as these theories were, it was somehow better than other theories, such as that of 17th-century English scientist Charles Morton, who believed that they flew to the Moon for winter, flying for 60 days at 201 kilometers per hour (125 miles per hour). To be fair to him, why bother going to the trouble of finding out things through empiricism in a century where you can merely say "whither should these creatures go, unless it were to the Moon?" and still retain your position as a respected man of science.
Weirder still was the notion – referenced in Homer's Iliad and later discussed as established fact by Pliny the Elder – that every year, cranes would fly south to continue their ongoing war with "pygmies", after a nice long break from the violence. Pliny wrote that the "pygmies" would ride out on sheep to attack the cranes and eat their eggs, in order to keep the population down, while the cranes would attack them back in their vicious war (note: don't assume a war on birds is a war humanity can win).
This is all to say that Europe was flailing pretty badly in regards to the mystery of where birds go when they leave during the colder months.
But then an explanation fell from the sky. With a massive spear right through its neck.
In 1822, near the German village of Klütz, a white stork was spotted, hanging out with a 76-centimeter-long (30 inches) explanation going right through its neck in the shape of a spear. The spear was found to be made of African wood, confirming that the stork had managed to fly 3,200 kilometers (2,000 miles) to Africa for the winter before making the immense return journey to Germany, where it was promptly killed and stuffed.
Further storks with spears through them have been found, which the Germans have christened Pfeilstorch or "arrow stork". Though it's probably of little consolation to a bird that was severely injured then made an epic journey only to be executed upon arrival, it proved a minor theory of the time about migration, putting to bed the frankly baffling ideas that they merely morphed into other birds for the new season.