Hitchcock might have choreographed the scene of death: it is midnight on a headland at the south-western tip of Skokholm Island, off the coast of Pembrokeshire. Slowly revolving beams from an automatic lighthouse droop like the rotors of a helicopter. The inky drizzle is filled with the wings of incoming sea birds, and from time to time there is a flash of silver as one is trapped for an instant by a relentless beam.
On an escarpment behind us a sinister welcoming party of great black-backed gulls is squawking in anticipation of a winged takeaway. In an effort to avoid such a fate, the Manx shearwaters have evolved the ploy of returning home under cover of darkness after fishing expeditions that might have taken them as far afield as the Bay of Biscay.
Sadly, it is not universally effective, and from time to time the night is riven by a piercing scream as a gull gets lucky in this life-and-death lottery. It is said they can turn the smaller bird inside out with a flick of their strong necks. Morning will reveal the night's tally of shearwaters in a scattering of carcasses over the clifftops.
Though wonderfully aerodynamic in flight, the bird's anatomy is not ideally suited to life on land. Its feet seem set too far back on its body, so it can only scrabble clumsily on its belly towards the subterranean safety of its nest. Shearwaters live in clifftop rabbit burrows, from which they evict the previous tenants each spring on arrival from wintering off the coast of Brazil.
Along with neighbouring Skomer, this island hosts the world's largest known concentration of the species, with almost 200,000 breeding pairs in occupation. Both islands are administered by the Wildlife Trust West Wales, which also runs the Welsh Wildlife Centre on the River Teifi at Cilgerran, near Cardigan. The trust will arrange a five-day package involving visits to all three centres.
Our hosts warned in advance of the vagaries of the Celtic climate in April. In the event, the only protection that proved indispensable was sunblock. Our arrival on the island aboard the ferry Dale Princess had been observed with bored indifference by three portly grey seals lolling in the morning sunshine on rocks near the tiny landing place. On the slipway to greet us is Graham Thompson, the island's knowledgeable warden. When the ferry leaves for the half-hour return trip to Martins Haven, we find ourselves alone with the seals, the birds, Thompson and his handful of staff. Before the advent of the mobile phone, our sense of solitude would have been complete.
The accommodation is comfortable, but basic. Lighting, cooking and refrigeration operate on gas, and the warden's computer is powered by the sun and wind. Drinking water is pumped from a spring, and running water for washing, bathing and showers is the kind of luxury only Sue Lawley could promise on Desert Island Discs . The less than hi-tech privy is only partially compensated for by the quality of bird art painted on its walls by previous occupants.
Presiding over mealtimes in the attractive dining hall is Alice Williams, the busty figurehead of a ship that foundered on the island in 1928. It is said that coal from the wreck kept Ronald and Doris Lockley warm for five years. Ronald had asked her whether, if he could build her a house in a year and a day, she would marry him. He could and she did. They farmed the island, and in the Thirties he established Britain's first bird observatory.
Only weeks after reading the glowing obituaries that appeared on his death in New Zealand at 96, it is moving now to sit in that house he built, and read a signed copy of one of the more than 20 books this early ecologist wrote.
Meals are cooked by Theresa Purcell, the warden's partner, and guests are expected to help with the washing-up. Long before we have finished a delicious dinner of fish pie and baked apple, gulls are gathering noisily on the roofs of farm outbuildings in another scene reminiscent of Alfred Hitchcock's Sixties chiller, The Birds. The contents of a bucket filled with the day's left-overs and emptied by a volunteer on a lawn vanish instantly in a flurry of wings and stabbing beaks. 'Our waste-disposal unit,' he says.
The island has no drinks licence, but guests are welcome to bring their own. Now, over a glass of wine it is time for the umpteenth retelling of the tale behind a trio of mysterious gravestones we have observed on the cliffs. They mark the last resting places, says the warden, of three lighthouse keepers murdered by an outraged Chinese cook who awoke one night to discover they had cut off his pigtail during a drunken spree. Sceptics' attention is drawn to the pigtail still hanging above our heads.
Violence is not unfamiliar to the island. Vikings used Skokholm as a springboard for their raids on the Pembrokeshire coast in the tenth century. Its ancient Norse name translates as Wooden Isle. Through the year its treeless landscape is bathed by successive washes of colour, as bluebells are succeeded in turn by red campion and bracken.
In the absence of land-based predators on Skokholm, rabbits introduced centuries ago threaten to undermine the entire island with their burrows. Yet, if they were wiped out, the whole ecology would change.
Vertical cliffs, like great apartment buildings, provide layers of accommodation for various species of birds in separate horizontal colonies. Pecking orders don't come more literally. Noisy kittiwakes occupy the basement ledges. Above them are the razorbills (thugs in dinner jackets, as our guide dubs them). Higher still, guillemots settle for the overcrowded ledges, and puffins sun themselves in their clifftop penthouses.
If the island has an emblem, it is the puffin. This winged comic with the Charlie Chaplin gait looks like a kind of parrot with the colour contrast switch turned low. Like the shearwater, it inhabits clifftop burrows. Its has to flap its economy-size wings frantically to fly, but it can swim underwater like a fish.
Craftily, the neighbouring guillemot lays a pear-shaped egg that is less likely to fall from its precarious lodgings. Three-week-old chicks hurl themselves, kamikaze-fashion, from the ledges into the breakers below and swim away to sea, only to return years later to breed within feet of their birthplaces.
Our party includes a couple of serious birdwatchers, for whom this trip must be a preview of the promised land. They came expecting to see the full range of sea birds, from cormorants to kittiwakes, but are surprised by the bonus of a variety of land birds, such as swallows, skylarks and chiffchaffs. Each night in the library, the warden holds a roll-call of the day's sightings of rarer birds.
It is said that when ornithologists took a Manx shearwater from its breeding site on Skokholm and released it in Massachusetts, the enterprising bird was back with its mate and chick in 12 days. There are recorded instances of human visitors returning to this magical isle year after year with similar dogged loyalty. It is not difficult to see why.
Three nights' full board on Skokholm costs £159 per person (seven nights £259), including ferry and landing fees from the Wildlife Trust West Wales on 01437 765462. The Posthouse Hotel, Dale, (01646 636201) is recommended for overnight stays on the mainland. B&B in a double room costs £48. Information from the Wales Tourist Board (029 20 49990 9)
