There are no records of Otter, Hedgehog, Mountain Hare, Black Rat, Brown Rat, Fox, Stoat, Ferret-polecat or Mink – all of which have been recorded on Shetland.
Feral cats are frequently encountered along the cliffs, but the size of the population and their effect on the local ground-nesting seabirds and land birds is currently unknown. Species such as Storm Petrel, Black Guillemot, Puffin and Arctic Tern are the most likely to be affected. There are also numerous domestic cats on the island, which inevitably take their toll on the influxes of tired migrant birds.
Despite Fair Isle’s vulnerability to colonisers, it does remain mercifully free of certain less than welcome biota (from a human standpoint): no wasps, no mosquitoes and no horse flies, while external parasites to bother the animals (such as sheep ticks) are infrequent.