bernaca

The barnacle goose (Branta leucopsis) breeds in the Arctic, far from medieval European eyes. Since no one ever saw their nests or eggs, a theory developed: the geese must grow from the goose barnacles (Lepas anatifera) found clinging to driftwood and ships. Gerald of Wales described this in his Topographia Hibernica (1187), claiming he had seen half-formed birds emerging from shells in Ireland.

The medieval Latin word bernaca (or barnaca) was applied to both creatures—the crustacean and the bird. The resemblance was not entirely crazy: goose barnacles have long, dark stalks and feathery feeding appendages that, if you squint, look like a bird emerging from a shell. The theory persisted for over four hundred years.

The theological implications were significant. If barnacle geese hatched from shells rather than eggs, were they fish or fowl? Some Irish clergy argued they could be eaten during Lent (when meat was forbidden but fish was allowed). Pope Innocent III settled the matter at the Fourth Lateran Council in 1215, ruling that barnacle geese were indeed birds and could not be eaten during fasts.

Science eventually killed the myth—the barnacle goose's Arctic breeding grounds were discovered in the 17th century. But the name stuck to both creatures. The crustacean is still called a barnacle, and the goose is still the barnacle goose. A medieval misunderstanding fossilized in the dictionary.