There are only two native land mammals in the Outer Hebrides, red deer and otter. The rabbit, blue hare, hedgehog, brown and black rat, feral cat and polecat were introduced by man. The origin of mice and voles is uncertain. The only reptile is the slow-worm, a legless lizard. No native amphibians are known and any frogs, newts or toads found have been introduced. Pipistrelle bats occur in and around Stornoway.
Marine mammals include the grey and common seal which breed on offshore islands, and many species of whale, dolphin and porpoise. Occasional turtles, mainly loggerhead and leathery, may be met in coastal waters.
Freshwater fishes present include salmon, sea trout, brown trout, Arctic char, rainbow trout, European eel, 3 and 9 spined sticklebacks, thick-lipped mullet and flounder.
Three hundred and twenty seven species of birds have been recorded in the Outer Hebrides and more than 100 breed. The Islands provide a natural flyway for migrating landbirds to and from their Arctic breeding grounds and a refuge for windblown vagrants from America and northern Europe. Many species of bird breed in the Outer Hebrides and the surrounding islands, including, most of Britain's Corncrakes which breed on the croftlands of all the islands.
Golden Eagles inhabit the uplands of Lewis, Harris and North and South Uist. A few pairs of Peregrine Falcons survive on coastal cliffs and Merlin and Buzzard are not uncommon anywhere on hill and moor. Owing to the absence of voles and other suitable prey in sufficient quantity in Lewis and Harris, Hen Harriers, Long and Short-eared Owls and Kestrels are confined to the southern isles. Red-throated Divers nest on countless small tarns and a very few Black-throated Divers on bigger lochs, mainly in Lewis. Arctic Skuas are spreading south from Lewis as far as South Uist but the Great Skua is still found breeding only in Lewis, St Kilda, North Rona, the Shiant Isles and Barra Head.
An important feature of the winter bird life is the great diversity of wildfowl. A variety of duck, such as Eider and Long-tailed are found in the shallow water around Lewis. The Uists and Benbecula are noted for their breeding Mute Swan and Greylag Geese as well as Whooper Swans and White-fronted Geese and duck.