During the Middle Ages, it was popularly believed that birds known as barnacle geese were not hatched from eggs but from the barnacles that often appear on rotting wood and on the undersides of ships.
In 1186, Giraldus Cambrensis, in his Topographia Hibernicae, described birds he called Bernacae. These bernacae were similar to marsh geese, though somewhat smaller. They begin their lives "like gum" (I am assuming he means that the birds start off as a sticky substance on the wood), and then progress to something of a larval stage where they hang from the wood by their beaks, ecapsulated in a shell-like covering.
The barnacle geese were said to have extracted nutrients from the sap of the wood they adhered to during development. At maturity, the birds would drop into the sea (to drown?) or fly away.
The theory of barnacle geese may have been partly the result of people wishing to circumvent the fasting requirements dictated by religious tradition. Priests and bishops in twelfth-century Ireland had no problem dining on various smaller seabirds during Lent; after all, the birds "are not flesh, nor born of flesh".
Reference: The Encyclopedia of Superstitions,
by E. & M.A. Radford, Dufour Editions, Inc. 1948