In ancient times, people didn't understand that birds migrate through the seasons. They just knew that some species disappeared and then popped up again in a different part of the year. Some written accounts hold some rather fanciful explanations for that, including the origin of the barnacle goose, which grew from floating logs. Gerald of Wales wrote, in the 12th century,
There are likewise here many birds called barnacles, which nature produces in a wonderful manner, out of her ordinary course. They resemble the marsh-geese, but are smaller. Being at first, gummy excrescences from pine-beams floating on the waters, and then enclosed in shells to secure their free growth, they hang by their beaks, like seaweeds attached to the timber. Being in progress of time well covered with feathers, they either fall into the water or take their flight in the free air, their nourishment and growth being supplied, while they are bred in this very unaccountable and curious manner, from the juices of the wood in the sea-water. I have often seen with my own eyes more than a thousand minute embryos of birds of this species on the seashore, hanging from one piece of timber, covered with shells, and, already formed. No eggs are laid by these birds after copulation, as is the case with birds in general; the hen never sits on eggs to hatch them; in no corner of the world are they seen either to pair or to build nests.
This turned out to be very convenient for the Catholic Church, as the barnacle goose could be classified as something other than meat, and therefore eaten during certain fasts. Learn about the barnacle goose and the real story behind it at Amusing Planet.