Hi, we recently had something similar happen with our ducks - something pulled all the feathers off (I've got a thread in the Ducks section of the forum). If it wasn't for the borken neck I would have suggested a rat was doing it. Would fit the size requirements....
Fishers are part of the weasel family as well as ermine and mink. They prey on various animals, some overlapping but their methods are similar if not the same.
I am thinking an overly aggressive rooster, especially if they are young. They will rip the feathers off the neck and break a hen's neck trying to breed with them. If he was in the nest box, he may have slammed her neck on the edge of the opening.