My first birdwatching kit was simplicity itself. Binoculars that worked quite well on sunny days, and a bird book with some artistic but inaccurate portraits of British birds.

Today I would be ashamed to go near the local reserve with anything so modest. Birdwatching has gone from small-scale hobby to multi-million pound industry in 20 years - and along the way it has managed to shed its image of being a nerdy pastime for would-be trainspotters in camouflage jackets. Now you need designer waterproofs, hi-power scopes and 2kg binoculars, the "designer jewellery" of the pastime.

There are an estimated 2.2 million birders in Britain, of whom about 150,000 are hard-core enthusiasts who spend up to pounds 2,000 on the kit - binoculars, scope, books - and often pay pounds 3,000 or more for guided trips to exotic locations. They are the subject of intensive advertising that supports several birding magazines, shops and an annual fair. Good bird books sell more than a million copies.

This weekend more than 30,000 birders in the UK - along with those from 77 other countries - will take part in World Birdwatch. The RSPB is running the British activities at 140 sites around the country, putting the spotlight on the beauty and value of birds. They are also trying to dispel the traditional view of the obsessive twitcher: the solitary, strange and inadequate man lurking beside the reservoir.

Experts such as Dave Cromack, editor of Bird Watching magazine, say that that image just isn't true any more. The birdwatcher's wife is no golf widow. She is likely to share in the hobby, which has the great advantage that you get better at it as you get more mature.

The dedicated collector of species will hire a helicopter or charter a boat to catch sight of the latest rarity to be blown to British shores. Hundreds of 'scope-toting fans turn up when an American warbler lands in a Surrey car park or an obscure Asian wader takes up temporary residence on a Hebridean islet. The man who has seen the most birds in the British Isles is Ron Johns, of Salthouse, Norfolk, with 502 on his list.

But the damage to farm or garden when a five-alarm twitch turns up can be devastating. When a houbara bustard landed in the east of England a few years ago, the farmer on whose fields it took up residence was nearly bankrupted by the trampling of his crops.

Birding literature is full of mini-dramas starring rare creatures. Twitchers have pursued an American thrasher into a loo in the Scillies, where it drowned; and stared solemnly for a day at a night heron on a Midlands marsh, which turned out to be a stuffed example hoist into a tree by a prankster. More than once they have seen their star rarity killed and devoured by some slightly less rare hawk or owl.

On a Shetland island, watchers photographed a Scops owl for days, until it quietly dropped off its perch and expired. They then had several months of arguments about whether it was "genuine".

The cognoscenti particularly enjoy these esoteric arguments about the likelihood of an owl or a marbled teal being a genuine thousand-mile migrant on a south-easterly airstream or an escapee from a Kent aviary two miles away.

At all levels it is a classless and good-natured hobby. Chris Meads, of the British Trust for Ornithology in Thetford, Norfolk, says that it is the continuation of the Gilbert White tradition - "people of a certain class, the squire and the parson, started our interest in local natural history and that spread down to teachers and pupils".

When the first field guides were published about 40 years ago, the RSPB's membership was 7,000. It has gone up a hundredfold. The RSPB admits that many of its members would rather watch wildlife on television than put on their wellies and go out into the woods to see real birds, but they generate a lot of money.

The main objection to birding is that some birders are obviously just out-of-context stamp collectors. On a recent trip to Moravia, I was with some twitchers who refused even to focus their binoculars on some wonderful wild ibex because "they are not birds".

But if even 10 per cent of birdwatchers are hoisting in some awareness of the natural world, of ecology and even rural manners, that is a bonus. As biologists say: it may not be a good thing, but it's not bad either.