Barnacle geese (Branta leucopsis) are medium sized Arctic Geese which spend their summers on the Arctic islands of the North Atlantic (in places such as the Northern coast of Greenland or the Svalbard archipelago). In winter, these places become utterly uninhabitable (actually they strike me as uninhabitable in summer as well, but I am a tropical hominid). The geese fly south to spend winter in the soft warm lands of Scandinavia, northern Europe, Scotland, and Ireland.
To the medieval inhabitants of these regions, barnacle geese were a mystery. They arrived fat and numerous in the coldest time of year, in the northernmost parts of Christendom. The geese breed in their summer ranges, so nobody other than Sami and Inuit had ever seen them nesting. They just showed up with more geese every winter.
Unfortunately the natural historians of the day tended to be of the “sit in a library drinking wine and making things up’ school of thought. Instead of renting an ice-breaker and following the barnacle geese to Svalbard, the scholars of the day just assumed the geese spontaneously generated from driftwood.
To quote the not-very-accurate twelfth century Welsh chronicler Giraldus Cambrensis (a Welsh-Norman archdeacon who wrote the Ferrebeekeeper of his age):
Nature produces [Bernacae] against Nature in the most extraordinary way. They are like marsh geese but somewhat smaller. They are produced from fir timber tossed along the sea, and are at first like gum. Afterwards they hang down by their beaks as if they were a seaweed attached to the timber, and are surrounded by shells in order to grow more freely. Having thus in process of time been clothed with a strong coat of feathers, they either fall into the water or fly freely away into the air. They derived their food and growth from the sap of the wood or from the sea, by a secret and most wonderful process of alimentation. I have frequently seen, with my own eyes, more than a thousand of these small bodies of birds, hanging down on the sea-shore from one piece of timber, enclosed in their shells, and already formed. They do not breed and lay eggs like other birds, nor do they ever hatch any eggs, nor do they seem to build nests in any corner of the earth.
It seems that poor Giraldus was taken in by observing barnacles on driftwood. He did not trouble to ascertain that the tenacious crustaceans never actually turned into geese. Interestingly/stupidly, the English name for barnacles is derived from the popular barnacle goose. The myth of the barnacle goose’s bizarre underwater larval parthenogenesis was of tremendous interest to medieval churchmen since it meant that the birds were not a prohibited food on various fast days. Irish and Scottish clergymen would not eat meat on Lent by enjoying sumptuous goose dinners! Pope Innocent III however was not swayed by the misinformation about the birth of barnacle geese and in the Fourth Council of the Lateran (1215 AD) he explicitly forbade the consumption of barnacle geese on fast days.
The actual egg-laying and birth of barnacle geese is nearly as harrowing as the medieval legend. In order to avoid the arctic foxes and polar bears of the northern islands, barnacle geese nest on jagged cliffs. When the goslings hatch they do not have wings, but they must jump down from these high cliffs onto sharpened rocks in order to reach the grasslands and wetlands where they can feed. Many of the fuzzy little goslings suffer, um, mishaps during this process (which sounds like the high point of the year for the arctic fox). This bloody rite of passage has however benefited the barnacle geese in the modern world. The sharp cliffs of remote Arctic islands are so unappealing that humans have not gone there to build ugly subdivisions or plant soybeans. In our world of extinctions and endangered animals, barnacle geese are doing just fine and are not even remotely threatened.