Today's Medieval Monday (Well Tuesday now by a few minutes!) comes from Gerald of Wales' History and Topography of Ireland, written circa 1185.
Barnacles that are born of the fir-tree and their nature.
"There are many birds here that are called barnacles, which nature, acting against her own laws, produces in a wonderful way. They are like marsh geese, but smaller. At first they appear like excresences on fir-logs carried down upon the waters. Then they hang by their beaks from what seems like seaweed clinging to the log, while their bodies, to allow for their more unimpeded development, are enclosed in shells. And so in the course of time, having put on a stout covering of feathers, they either slip into the water, or take themselves in flight to the freedom of the air. They take their food and nourishment from the juice of wood and water during their mysterious and remarkable generation. I myself have seen many times and with my own eyes more than a thousand of these small bird-like creatures hanging from a single log upon the the sea-shore. They were in their shells and already formed. No eggs are laid as is usual as a result of mating. No bird ever sits upon eggs to hatch them and in no corner of the land will you see them breeding or building nests. Accordingly in some parts of Ireland bishops and religeous men eat them without sin during a fasting time, regarding them as not being flesh, since they were not born of flesh.
Hmmm... a crafty way to get around the meat eating regulations methinks! But that is why barnacle geese are so called....