The mammals that have naturally colonised the Northern Isles, the Inner and Outer Hebrides are limited; pygmy shrews, field voles, Orkney vole, mice and rats, otters, red deer and seals. It is probably the absence of land predators, including foxes and stoats, that has resulted in such high numbers of ground-nesting birds, such as waders and corncrakes. Feral cats can be a nuisance, as can a more recent introduction, ferrets. They were originally brought to the island to control the rabbits, themselves imported by humans, as a potential source of food. Tiree and Berneray (off North Uist) still have no rabbits although the latter will be at risk of invasion over the new causeway.
There is no justification for introducing any wild animal to an island. Ferrets have never succeeded in controlling rabbits. Rats, cats and mink have wiped out many bird colonies. Mink reached the Uists from Lewis and Harris in the mid 1990s and are spreading. Hedgehogs – introduced to the Uists in the 1970s – are not munching as many garden slugs as they were expected to, but also find wader eggs appealing. Recent studies by SNH and the RSPB have demonstrated serious declines in wader numbers in South Uist and Benbecula due to hedgehog predation, while North Uist too is now being invaded by this prickly predator; they are also present in Coll and Tiree. Natura designations in Uist have drawn substantial European funding to protect the internationally important concentrations of breeding waders from mink. A trapping programme is now under way and it is hoped that the hedgehog problem can be similarly addressed.