Are they not on some Scottish island though, where there no wild polecats.
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found this
Colonies of feral ferrets have established themselves in areas where there is no competition from similarly sized predators, such as in the Shetland Islands and in remote regions in New Zealand. Where ferrets coexist with polecats, hybridization is common. It has been claimed that New Zealand has the world's largest feral population of ferret-polecat hybrids.[16] In 1877, farmers in New Zealand demanded that ferrets be introduced into the country to control the rabbit population, which was