British Wildlife of the Week
St Kilda. A remote, windswept archipelago in the storm-tossed seas 160 kilometres west of mainland Scotland. This scattering of rugged islands is famous for its birdlife. Nesting on and around its sea cliffs – the tallest in Britain, up to 426 m – are a huge variety of seabirds, including fulmars, gannets, puffins, guillemots, storm petrels, razorbills, shearwaters and skuas. It was also the final British stronghold of the great auk; the last one was seen and killed here in 1840, just four years prior to its global extinction.
The largest of St Kilda’s islands is Hirta, the only one that has been populated by people in recorded history. Its human inhabitants lived there for thousands of years until illness and hardship reduced their numbers to the point that they could no longer sustain a permanent community. In 1930, at their own request, the last 36 islanders were evacuated to the Scottish mainland. Today, all that remains of this lost civilisation are igloo-like stone structures called cleits, which the islanders used to store the corpses of seabirds during the winter months, and a row of tumbledown houses.
To the northwest of Hirta lies the tiny isle of Soay, only a mile across. It is home to a supposedly wild sheep breed, which bears the same name as the island. In truth, these Soay sheep are not truly wild; they’re the feral descendants of sheep that were once, thousands of years ago, domesticated. Because they have remained virtually unchanged for all that time, Soays are essentially living relics from ancient times, showing us what domestic sheep looked like a few millennia ago. They are smaller and daintier, with dark fur and curved horns – quite unlike the fat, white woollies we normally see today.
Soay sheep do not originate from the island of Soay. In fact, this breed was once widely distributed across much of Neolithic Europe. Perhaps Bronze- or Iron-age pioneers carried them to this far-flung archipelago when they settled there long ago. Certainly, by the time Vikings arrived and named the island, the sheep were already well-established, for ‘Soay’ comes from an Old Norse word (Sauda-ey) meaning ‘Island of Sheep’.
Elsewhere in the world, farmers began selectively breeding sheep to make them more useful to us. As the centuries and millennia passed, breeds became more specialised for their meat and wool. The latter involved the production of white fur, which is easier to dye, and consists only of the curly underfur of a typical mammalian coat. The long guard hairs have been bred out of most modern sheep as they interfere with the production of strong wool fibres. And so the original sheep breeds largely disappeared, being displaced by, or incorporated into, more ‘improved’ breeds.
However, because of Soay’s remote location (it’s the westernmost point in the United Kingdom), the sheep that lived there had little opportunity to mix with any other breeds. Unaffected by trends in modern sheep breeding, these small, horned sheep are, as far as appearances go at least, pretty much the same as those tended by Bronze Age shepherds more than 4,000 years ago.
Soay sheep are hardy and extraordinarily agile. Having adapted to navigating the steep cliffs and rocky fields of their island, they tend to take refuge amongst the cliffs when frightened. They are capable of grazing in areas where modern sheep would struggle to survive, either because the ground is too soft for larger breeds or because the vegetation is too sparse and lacking in nutrients.
In 1932, two years after Hirta was abandoned, some Soay sheep were introduced to the island from Soay to form a second population. There, they have no predators (apart from gulls, ravens and hooded crows, which occasionally take very small or weak lambs), so there is nothing to regulate their numbers apart from the availability of food. Consequently, the population fluctuates seasonally and from year to year depending on the food supply.
Living on an isolated island for several generations can affect the physical size of an animal. Some grow larger to take advantage of an abundance of food; others are unable to find as much food and therefore become smaller. A subspecies of Eurasian wren that has evolved on St Kilda, for instance, is now bigger than its mainland relative, being a sparrow-sized bird, while the hardy St Kilda field mouse is almost twice the size of its counterparts in mainland Britain . The Soay sheep, however, are getting smaller. They are approximately a third of the size of most commercially reared sheep (females weigh about 24 kg; males, 38 kg), and four-month-old lambs are, on average, 80 grams lighter each year.
In the next British Wildlife of the Week, we’ll be looking at Britain’s only native venomous snake, and one of the most northerly reptiles in the world – the adder.
 Another mouse – a subspecies of house mouse this time – also used to live on St Kilda, but it became extinct almost immediately after the last remaining villagers on Hirta departed the island for good, having lived solely among them and subsisting on their leftovers.