Soay Sheep: The Wild Survivors of St Kilda

Each major claim carries one of three labels. established means supported by primary records, official bodies or several independent reliable sources. shared means the core fact is sound but sources disagree on details, dates or interpretation. traditional means widely repeated but not traceable to a primary or independent source.

The deep origins of the Soay are often stated confidently as Neolithic or Bronze Age, or as brought by Vikings, but the evidence is indirect. The research findings come from decades of peer-reviewed studies.

A living link to the first farmers

shared Soay sheep look strikingly different from modern breeds. They are small, slender and agile, usually dark brown with a pale belly, and most rams carry curling horns. Their fleece moults naturally each summer rather than needing shearing, and St Kildans traditionally plucked it by hand, a practice known as "rooing". Because they resemble the wild mouflon more than modern sheep do, they are often described as a survival of the sheep of Europe's earliest farmers. Their exact origins, and whether they reached St Kilda in prehistory or later, perhaps with Norse settlers, are uncertain.

From Soay to Hirta

established For centuries the sheep lived wild on Soay, a steep island of about 100 hectares next to Hirta, the main island of St Kilda. The St Kildans kept other sheep on Hirta and occasionally caught Soays. When the last 36 St Kildans left Hirta in 1930, the island was empty of people and their sheep. In 1932 the owner, the Marquess of Bute, had 107 Soay sheep moved from Soay to Hirta. Their descendants now graze freely around Village Bay, among the ruined blackhouses and cleitean.

Boom and bust

established With no predators and no human management, the Hirta sheep follow a dramatic cycle. Almost every mature ewe conceives each year regardless of how crowded the island is, so the population grows beyond what the winter grazing can support, and then crashes, sometimes losing more than half its members in a single hard winter. Crashes are most likely after harsh weather and when the population contains many lambs and young males.

A natural laboratory

established Because the population is isolated, unmanaged and confined to a small island, it has become one of the most intensively studied wild animal populations in the world. Counts have been made most years since 1952. The first research phase, from 1959 to 1967, led to the monograph Island Survivors (1974). In 1985 Tim Clutton-Brock and Steve Albon restarted intensive study, tagging every sheep in Village Bay, and the work continues as the Soay Sheep Project.

shared The research has contributed to understanding of population dynamics, parasites and natural selection, including how coat colour and horn type are inherited and selected. A widely reported 2009 study found that the sheep's average body size had declined over several decades, and linked this partly to milder winters allowing smaller, slower-growing lambs to survive.

What the 1991 Handbook recorded

established Who wrote it. The feral sheep account in G. B. Corbet and S. Harris (eds), The Handbook of British Mammals (3rd edn, 1991) was written by Peter Jewell, who had led the first Hirta study from 1959, and David Bullock of Leicester Polytechnic, who also wrote the Handbook's feral goat account. It is a summary of the first three decades of St Kilda sheep research, before the long-term project's later findings.

established What a Soay looks like. The Handbook describes the Soay as narrow-bodied, relatively long-legged, short-tailed and narrow-faced, closer in many ways to wild sheep than improved breeds are. Adults in good condition weigh up to about 35 kg for rams and 26 kg for ewes, and stand about 50 cm at the shoulder. Most rams and about half the ewes are horned; other ewes are polled or carry small, deformed horns called scurs. Colour runs from light fawn to blackish, with "light" and "dark" forms most common, and a whitish belly and rump patch like the wild mouflon. Unlike wild sheep, the coat is mainly wool, cast each summer.

shared Origins. Jewell and Bullock note that domestic sheep came to Britain with Neolithic farmers, and that the Soay's horn shape, short tail, colour and moult suggest an affinity with wild sheep. Its bones closely resemble sheep bones from late Neolithic, Bronze Age and Iron Age sites. But they add that a later introduction by Vikings or in Christian times is "equally likely". The Soay is best described as a survival of an early type of sheep, not as proof of a prehistoric arrival.

established On Soay and Hirta. For centuries the sheep were confined to Soay, where the population was estimated at between 150 and 300. Between 1952 and 1987 the Hirta flock fluctuated between 610 and 1,783 sheep counted in early summer, about 0.9 to 2.8 per hectare, with periodic die-offs linked to density. Most deaths come in February and March, when the grazing falls below what the sheep need. Rams spend so little time feeding during the autumn rut that they are especially vulnerable in late winter; on Hirta all rams were dead by ten years old, while the oldest ewe recorded reached 15.

established Family groups. Ewes on Hirta live in matriarchal "home-range groups" of about 27 to 37 sheep, while young rams leave by the age of two to join small ram groups. During the rut, rams wander widely in search of ewes, and high-ranking rams form more consorting bonds with ewes in season. Lambs, usually single, are born from March to May, with median lambing dates in mid to late April.

established Wild in behaviour. The Handbook notes that St Kilda sheep flee when approached within about 100 metres, though they become bolder with regular human presence, and that they scatter when worked with a sheepdog and cannot be rounded up that way. The sheep tick has never been recorded on St Kilda.

St Kilda's other sheep

established St Kilda also has a second feral breed, the Boreray sheep, on the island of Boreray. According to the 1991 Handbook, it represents the type of sheep the St Kildans themselves herded: a cross between the Blackface, which reached Scotland in the 18th century, and the Old Scottish Shortwool now represented by Orkney and Shetland types. The islanders could not gather their Boreray flock before the 1930 evacuation, and it has lived wild since. Boreray sheep look like small Blackface sheep, usually cream or off-white, often with a dark collar; all are horned, and rams' horns can reach 75 cm along the curve. In 1980 a land count found 699 sheep, about 12 per hectare, a very high density probably sustained by grass fertilised by seabird droppings.

established Both breeds were classed by the Rare Breeds Survival Trust as rare breeds, and both have been kept on mainland farms and in conservation grazing. Soays have also been introduced to other islands, with a well-known flock on Lundy in the Bristol Channel. Confusingly, the name "St Kilda sheep" was once used for the four-horned breed now called the Hebridean.

• Soay sheep shed their wool naturally and were traditionally plucked, not shorn.
• They are named after Soay, an island whose name means "sheep island" in Old Norse.
• 107 sheep were moved to Hirta in 1932, two years after the last people left.
• The Hirta population can lose more than half its members in one winter.
• They are among the most studied wild animals in the world.
• Soay sheep cannot be rounded up with a sheepdog; they simply scatter.
• About half of Soay ewes grow horns.
• The Boreray sheep are descended from the St Kildans' own flock, left behind in 1930 because it could not be gathered.
• The sheep tick has never been recorded on St Kilda.

• Disputed: When and how the Soay reached St Kilda. They are often said to be Neolithic or Bronze Age survivors, and their bones resemble those from late Neolithic to Iron Age sites. But the 1991 Handbook of British Mammals judged a later arrival with Norse settlers or in early Christian times equally likely. The evidence is indirect.
• Myth: The Soay is a wild species. It is a primitive domestic breed that has lived feral for centuries.
• Disputed: Why the sheep have grown smaller. A 2009 study linked the decline in average body size partly to milder winters allowing smaller, slower-growing lambs to survive. Climate is only part of the explanation.
• Myth: Soay, Boreray and "St Kilda" sheep are the same thing. The Boreray is a separate feral breed, a cross of Blackface and Old Scottish Shortwool descended from the St Kildans' own flock. The name "St Kilda sheep" was once used for the four-horned breed now called the Hebridean.
• Myth: The Hirta and Boreray sheep share an origin. The Soays were moved to Hirta from Soay in 1932. The Boreray flock is the St Kildans' managed stock, left behind in 1930.