Is it a bird? Is it a plane? No, it’s . . . a kittiwake. I think, but then again, maybe it’s a lesser black-backed gull. As it swoops and glides joyously on the thermal currents above Skomer Island off the coast of Pembrokeshire, it takes a practised eye and a pair of binoculars to tell the two apart.

Skomer is a long-dead volcano, whose whorled and twisted rocks and lava flows are now managed by the Dyfed Wildlife Trust as a reserve for breeding seabird colonies. Every day, visitors festooned with outsize telescopic devices pile on to its craggy shores. I joined a group of them for a day’s introduction to birdwatching run by Acorn Activities, which promised to initiate me into the arcane world of twitching.

There is something slightly eery and mysterious about birds, not least because of what Alfred Hitchcock did with them. Bird groupings, for example, have evocative names which defy explanation: a desert of lapwings, a murder of crows and a tittering of magpies. Eagles come in convocations, goldfinches a charm and larks an exultation. But how many of us would know a lark from a goldfinch, let alone an exultation if we saw one?

My guide through this ornithological twilight zone is Peter Brown, an ex-teacher and now freelance ecologist, who can spot a guillemot from a shag half a mile away. But before we are let loose on the island’s cliff tops, binoculars at the ready, the warden solemnly advises us not to stray from the pathways for our own safety. What kind of birds are breeding here, for goodness sake? The answer is subterranean ones. Many of Skomer’s inhabitants live in burrows and what look like inviting grassy knolls are in fact avian condominiums which will collapse like the proverbial pack of cards under the weight of anything heavier than a rabbit.

Although the boat which brought us here from the mainland was packed to the gunwhales, Skomer is big enough to accommodate all its passengers and the paths are soon pleasantly free of other watchers. The island is a gently undulating carpet of wildflowers, with white foaming sea campion, rust-red sorrel and water forget-me-nots providing a gorgeous sweep of colours and textures. Every so often a plume of sea fog rises over the cliffs like smoke, only to disappear just as suddenly under the bright summer sun.

Peter Brown gives more of a holistic introduction to Skomer than a checklist of which sea-birds live where and how their wing markings differ. This way, you learn about how the island has developed ecologically to become a sanctuary for so many species. Littered over the paths and pools are the carcasses of Skomer’s most populous inhabitants, albeit ones that 99 per cent of visitors never see. The world’s greatest col-ony of Manx Shearwaters are nesting in the condos beneath our feet and will only emerge at night. The puffins, by contrast are unself-conscious and easily identifiable. The beauty of Skomer is that you can observe them from startlingly close quarters and their take-offs and landings are richly comic to watch.

Correctly identifying the island’s other seabirds becomes much more difficult when you get into sub-species of gull and auk. Some differ only in the colour of their legs. Simple enough, you may think, but yellow and pink legs look disarmingly similar especially when the sun’s in your eyes.

Meanwhile Peter has something else in his sights. “You can always tell a gannet at a distance,” he says confidently, while staring out at the horizon where sky and water meet. You can? “They are intensely white”. But then as the majority of sea birds filling Skomer’s skies have at least some intensely white bit on them, this does not seem particularly helpful. But I stared hard, and skimming the waves in the far distance is a brilliant flash of the kind of whiteness Persil can only dream about. And I know what he means. It’s a gannet and you know when you have seen one.

Peter Brown’s own favourite is the great black-backed gull. “How can you admire a tiger or a lion, when the great black-backed gull is such a magnificent predator?” he asks (it is for this reason alone that the Manx Shearwaters are holed up in their burrows until nightfall). “I have seen one take a rabbit head first and watched its legs kicking as it disappeared down the gull’s throat”. Clearly not a bird to antagonise.

A day is not nearly long enough to get to grips with birdwatching. There is so much to see and remember that all it can do is engender a curiosity that may bring you back for more. So I saw razorbills and fulmars, and heard the huge voice of a tiny wren but I’m afraid I would no sooner recognise any of them than a goldfinch. But I do know the difference between a kittiwake and a lesser black-backed gull, I think . . .

The course cost Pounds 40 and was organised by Acorn Activities, which runs a year-round programme of special interest breaks. Acorn also arranged excellent accommodation at a local farmhouse. Acorn Activities, PO Box 120, Hereford HR4 8YB. Tel: 01432 830083.