See I love this though, because this is one of the few pieces of medieval natural history where, as batshit insane as it is, the reasoning behind it does actually make sense.
See, barnacle geese live in the UK, but they don't breed here. They travel to the arctic to have their babies and don't return until they're fledged.
So, from the medieval perspective— given that most medieval brits weren't really in a position to go down to the arctic and check out the wildlife— barnacle geese a) vanished every summer and b) didn't seem to lay eggs or have babies. They just showed up at the same time every year, already basically grown.
Enter the goose barnacle.
The goose barnacle, supposedly, has a similar pattern and colouration to the barnacle goose. I can't really see it myself, except that they both have white patches, but medieval people apparently could.
Crucially, however, the goose barnacle likes to attach itself to driftwood, meaning that branches would regularly wash up on UK beaches covered in them. People at the time (not unreasonably) assumed that this meant that the barnacles grew on trees, whose branches later snapped off and fell in the water.
So, we have all these geese who vanish every summer and then emerge later in the year as full grown adults. And we have all these barnacles that apparently grow on trees (where birds like to live) and have a similar colouration to the geese.
So like, it's insane from a modern perspective, but you can see how they got there.
But ALSO, because your barnacle goose was born from a barnacle, many people at the time argued that it was technically a fish in the eyes of the Catholic church. At a time when nobody was allowed to eat meat significant chunks of the year, this was a useful myth to believe in.
The church, incidentally, did not agree with it. Not because they rejected the idea that barnacle geese spontaneously generated from barnacles— they were all in favour of that— but because, in the words of one writer “if anyone were to eat of the leg of our first parent (Adam) although he was not born of flesh, that person could not be adjudged innocent of eating meat”.
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