Sources : Barnacle Goose

Gerald of Wales [c. 1146 – c. 1223] (Topographia Hibernica, Distinction 1, chapter 11): There are likewise here many birds called barnacles (barnacle geese) 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. - [Forester translation, 1863]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.23): Barliates, as Aristotle says, grow from wood, and are birds which the common people call 'barnesques', having a similar nature. And Aristotle says in his book De animalibus: a certain researcher saw in the sea an animal exceedingly wonderful, whose creation was like the creation of wood, and it was black, round, equal in thickness, and had many wings in it, that is, a multitude of winged birds. And it is said that when a tree is cut down by the waters of the sea, with the passage of time, when it has begun to rot, it emits a thick moisture from itself, and from that condensed moisture, small species of birds are formed, to the size of larks, and are naked at first. Then, when they are ripe, they pluck their feathers and, hanging their beaks on a tree, they float through the sea until they are ripe, until they break off, shaking themselves, so that they grow and become stronger until they reach their proper form. We have seen trustworthy men who have testified that they saw these birds still hanging on the tree, of which we ourselves have seen many. Having a gray and black color, they are smaller than geese; and they have feet like ducks, but black. Of these birds James the bishop of Acon, in his Eastern history, says not unlike the matter. For he says that there are trees about the coasts on the shores of the sea, from which birds are born, hanging on the trees with their beaks fixed, but after the time of maturity has arrived they immediately fall from the trees and (as they progress) begin to fly like other birds. Unless they find the waters quickly after falling, they cannot live; for in the waters is their nourishment and life. It should be noted about these birds that they do not hang on the top of branches, but on the bark and trunks of trees. And they grow by the infusion of the moisture of the tree and the dew, until they have feathers and the strength of life and break off the bark. And this is indeed certain of these birds, that in our country, around Germany, they neither reproduce by copulation nor are they begotten by copulation, but no man has ever seen them copulate among us. They live on herbs and grasses like geese, but they can in no way refuse to eat dry food. In Lent, some Christians used to use the meat of these birds around parts where there is plenty of birds, even in our age. But Pope Innocent the Third, in the general council of Lateran, forbade this to be done any longer. - [Badke translation/paraphrase]