In 1187, a Welsh Monk claimed to have witnessed the transformation of a certain type of barnacle into a goose, which then flew out of the water into the air.
It may sound like a crackpot idea, but at the time it was a common belief. Many thought that the so-called "Goose Barnacle," a crustacean found in the oceans around Northern Europe, was indeed the juvenile form of the Barnacle Goose, a large migratory bird. So the monk's claim wasn't too shocking at the time.
Back then, little was known about bird migration. The barnacle goose didn't breed in Europe and adult geese just seemed to appear each fall. Since no one had ever seen their eggs or babies, the arrival of the geese was a real mystery.
The myth that barnacle geese grew from barnacles was based on some coincidental similarities. The goose barnacle has a white beak-like shell that attaches to solid objects like driftwood or rocks. From this shell, the barnacle extends a long flexible brown "neck" used to filter bits of food from the water. The color of the barnacle and movement of its neck bear an uncanny resemblance to the barnacle goose's white face and long brown neck.
Using careful observation and small tracking devices, modern scientists now know quite a bit about these mysterious migrants.
Barnacle geese leave Europe in spring and fly north to breed on rocky Arctic islands. Newly hatched young and their parents spend the long summer days feeding on plants in the tundra. As the days shorten, they return south to wintering grounds in Northern Europe, where they stay until spring returns and the migration cycle begins again.