Barnacle geese breed in the high Arctic, on cliffs in Greenland, Svalbard and Siberia, and appear in western Europe only in winter. Before Arctic breeding grounds were known, the absence of eggs or goslings demanded an explanation. Gooseneck barnacles on stranded driftwood, with their dark feathery cirri and long stalks, resembled the birds enough to supply one, and the idea that the geese grew from the barnacles was repeated in serious natural history for centuries.
The belief had a practical use. Because the birds were held to originate from something other than flesh, they were in places treated as permissible food during fasts, an accommodation that church authorities eventually pushed against. Gerald of Wales recorded the practice in the twelfth century.
The real animals are unrelated. Lepas anatifera is a stalked crustacean whose larvae settle on drifting objects and feed by combing plankton from the water, while the barnacle goose, Branta leucopsis, is a bird whose migration was documented only once ringing and Arctic survey work confirmed it. The linguistic fossil survives in the species epithet anatifera, meaning duck-bearing.