Where sea birds cry: In an exclusive extract from his new book Islander, Patrick Barkham visits the evacuated archipelago of St Ki

The triangle of rock draws closer – an islet, Levenish, beside which emerges a much larger land mass: Hirta. In its shelter, the swell drops and we ease into Village Bay, a natural amphitheatre between two mountains, filled with buzzing insects that turn out to be puffins. The grassy slopes are covered in daisies, which are actually the heads of fulmars. The rocks all around are stained with guano as pale yellow as a gannet’s head. Not many species have colonised this remote mountainous island – just two human-assisted mammals and 180 flowering plants compared with 600 on the Isle of Skye – but St Kilda’s wealth comes from the sea. West of the British archipelago, the continental shelf drops away to deep ocean and up this edge rise nutrient-rich waters that support vast reserves of kelp, plankton, fish and seabirds. Humans once completed that food pyramid, and we are fascinated today by the Hiortaich’s unusual diet: not fish (the sea was usually too hazardous for fishing), but seabirds. A 1764 census recorded 90 islanders each devouring "36 wild fouls eggs and 18 fouls" daily, which I can’t quite believe, but other documents record that 89,600 puffins were slaughtered for meat and feathers in 1876 alone, and the typical adult ate about 115 fulmars each year. Puffin was boiled with porridge oats to liven up breakfast.

Today, the seabirds are unharvested and St Kilda is home to the

largest British colony of puffins and fulmars, the largest Leach’s storm petrel colony in the eastern Atlantic, and the second-largest gannetry in the world.

Wobbly with the swell and with gratitude, we are decanted into a little Zodiac – to minimise the risk of accidentally introducing rats to the island – and putter over to the small stone pier. We walk up the ramp at Village Bay, and our first vista is not of the ruined cottages that are St Kilda’s public image, but of a low mass of olive-green prefabs like the mobile classrooms of my childhood. It’s ironic that after the 1930 evacuation, the island was uninhabited for just 26 summers. This mountain in the Atlantic was too useful to ignore indefinitely. It was reoccupied in 1957 when the island was given to the National Trust for Scotland and the RAF built a concrete road zigzagging up the mountainside of Mullach Mòr to service a radar station.

The station is now operated by a private contractor, QinetiQ, which tracks missiles test-fired from Benbecula. An air-andsea danger area that extends 260 by 95 kilometres into the North Atlantic is cleared of boats whenever a missile needs firing. "Please don’t go in these buildings, you might never come out," says Kirsten Dallas, the National Trust for Scotland ranger whose job is to meet all arrivals. "We don’t know what happens in there." Between the low prefabs, ships’ containers and oil tanks is a taller chimneyed unit containing diesel generators that emit an incessant grinding hum. These are omitted from the tourist vision of St Kilda, but they sustain modern life here.

On the slopes above are the buildings that sustained the older life: dotted all over the hills are hundreds of cleits, windowless huts of tapered stone with green turf roofs: storehouses for fish, turf and everything else that got the Hiortaich through the long winters. Their curved walls remind me of the beautiful beehive structures I’ve seen in western Ireland, built by Celtic monks. In Ireland, some of these clocháns have been used into modern times, and the St Kildan cleits were deployed until the islanders’ hour of departure. Beside and inside them tread Soay sheep, a dainty brown breed probably introduced by the Vikings. Gaggles of lambs gather on the cleit roofs, dancing around like teenagers at a party. Since the evacuation, these beasts have roamed wild, munching Hirta’s heather and orchids into permanent miniature. Unlike most domesticated breeds, they shed their fleece naturally; during their late-spring moult, the Hiortaich simply pulled at their ginger-brown dreadlocks to obtain soft, waxy wool, a process known by the lovely word, "rooing".

Day-trippers fan out in a frenzy along the elegant crescent of single-storey stone cottages built in a style familiar across Scotland and Ireland’s west coast. The trippers are limited to four hours on the island, but as a camper I have the luxury of time. The cloud lifts and I erect my little tent in the empty camping field and hang my food in the cool interior of a cleit, surprised to feel a breeze wafting through cracks – a deliberate design to dry whatever is stored inside. I hope my hung food will evade the attentions of the island’s third mammal (after sheep and humans): the St Kilda field mouse. When the people left, the house mouse quickly died out, but the more independent field mouse thrived in its niche.

The first I see of another islander is the vermilion insides of its

mouth. Singing loudly, bouncing between the cracks of a cleit, the St Kilda wren is sparrow-sized and brightly speckled, having evolved into its own subspecies during its time here. Assured, noisy and distinctively large, the wrens hold Village Bay in a series of territories.

Islands are places of giants and pygmies and the wren is evolving to be larger, while the sheep get smaller. Would this wren’s descendant, like a mighty eagle, one day clasp sheep in its claws? Scientists studying the Soay sheep have linked their shrinking to warmer winters: they no longer need to be so bulky to survive.

The wren is accompanied by the retching-into-a-tin-can chat of

the fulmars. The epitome of elegance in flight, wings spread so flat they look like gliders, they are less graceful on land, squatting on rocks around Village Bay, chuntering and complaining. Author Compton Mackenzie reckoned they possessed a cold disdainful eye"; he preferred kittiwakes, "the prettiest and daintiest of all the gulls".

The trippers depart and I join the fulmars, sitting on a rock, surveying the scene. I’m rendered motionless by an evening that unravels like tissue paper, becoming softer and softer. The land darkens, but the sea shows all patterns of light. Silver traces of current wiggle like snail trails over the calm Atlantic. The sun spins cumuli into gold and the clouds cast golden shadows on the blue water. The gold turns pink, purple and then the palest of grey. I feel a great weight pressing upon me and submit to it, long before the dusk has darkened.

Soft rain wakes me soon after 5am, and I wander the ruins of Village Bay. Later I make a day-long circumnavigation of Hirta, feeling my way along its edges. Periodically, I drop to my knees and peer over the cliff, grey-green lichens as prickly as a doormat under my hands. A kaleidoscope of fulmars turn against the dark water, crying over the white-noise menace of surf meeting rock a stomach-lurching distance below. The cliffs aren’t always sheer, but I can no more scale them than fly like a fulmar. We mainlanders live on the flat. Land is horizontal. Here, half the island is manifest in the vertical. If your terrain is half-cliff and you don’t climb, you are only half alive. And the Hiortaich climbed to stay alive. "The inhabitants look as if they had been all tarred and feathered, for their hair is full of feathers and

their clothes are covered with feathers," wrote John MacCulloch in 1819. "Everything smells of feathers."

A sustainable harvest of seabirds supported life on St Kilda. Gannets, or guga, were a staple of earlier times, but after the 1750s the islanders ate more fulmars, which provided fatty meat said to taste like beef. Fulmars – "foul gull" for their habit of vomiting a stinky fluid over intruders – also provided an oil sold on the mainland as medicine, reputed to possess similar properties to cod liver oil.

Another cash crop was fulmar feathers, which the British army used as lice- and bedbug-resistant bedding in the 19th century.

Tough seabird flesh was observed to be good for the islanders’ teeth.

They also ate plenty of eggs, particularly gannet and guillemot, often keeping them for six to eight weeks to improve their flavour. Other species – oystercatcher, tree sparrow and especially the egg of the St Kilda wren – were blown and sold to collectors. I admire the guillemots huddled on ledges, their backs turned to the water.

There is a giddy experience to be had in wild places, which Alasdair Gray in Lanark describes as being "drunk with spaciousness". It is intoxicating to walk along An Cambir, the north-western arm of Hirta, where it stretches towards the uninhabited island of Soay, and it is also invigorating. I share a drunk’s exaggerated sense of his own reach, but I imagine I can see and feel with new precision, which is not a characteristic of the inebriated. I have the light, the air and the landscape to thank. Compton Mackenzie wrote of the "clarity" of Hebridean days when "the islands seem to float suspended between earth and heaven

in a crystal globe".

I climb to the highest point of Hirta, above the three white eggs

in concrete cups belonging to the radar station. They could be an art installation, an aesthetic response to the eggs all around us. The last cloud vanishes as a stage curtain lifts, and suddenly is revealed the full grandeur of this world. I can only gasp at its splendour. A vast expanse of empty sea glitters for miles around. Then, along the eastern horizon emerges a chain of islands. In the foreground, the low islets of Haskeir.

Beyond are the mountains of Harris, and the hummock-backed hills of North Uist, Benbecula, South Uist. Further away to the south, a paler grey, is Barra. Beyond it, perhaps Vatersay or even Mingulay. I can’t see another soul.

This is an edited extract from Patrick Barkham’s Islander, published by Granta Books on October 5. He is appearing at Wigtown Book Festival at 10.30am on Saturday, September 23. See www.wigtownbookfestival.com for tickets.