dobesgalore
you aren't really familiar w/ ferrets are you. the ones in the pet shop are babies. a grown ferret is 3-4X bigger than weasels that regularly kill adult chickens. in fact they are about the size of small housecats. a grown hob w/ good hunting instincts would ravage the average chicken. also as a predator they are naturally more active at night when ground & shrub roosting birds are most vulnerable. ferals in britain regularly take small birds and make occasional forays into coops & lofts after hens & pigeons.
while most wouldn't do well here because our rabbits don't warren, some do have all the tools they need to survive long enough to procreate if given the chance. don't let the clowning antics fool you.
Our old ferret was huge, but a giant wuss. He used to let the house mice come in his cage and eat his kibble.
Surely there are a few out there, but not enough to warrant a ban on ferrets as a danger to anybody's livestock or wildlife.
The other big thing about ferrets is 90% of the ferrets sold in the pet trade are already altered. I think the breeding of them is regulated by the USDA, like with hedgehogs.
We did have a pair of euro mink come in and ravage the chicken coop, lost 17 hens in one night. It was devastating. They tried to get into the pullet coop next, but we had old Lefty in there. He was the biggest best araucana roo ever. He lost his wing to a coyote years before. Anyway, he piled drived one mink to death with his stub (that's how he beat the crap out of the other roosters too) and when we caught the second mink it had a broken jaw, probably Lefty's work as well. I wish he had been in the hen house that night. Those mink were the nastiest orneriest foulest things ever. The second one we caught
in our house, trying to get into the brooder with chicks, so we caught it in a laundry basket as it tried to attack us. I won't post what happened to it after that.....