You may not think much of the British winter, but they come in their millions to seek it. There is an annual pilgrimage from across the world to the land of the wet and the chill: and what's more, they seek the wettest and chillest places we can offer. So the good barge Victor pulled away into the Stour estuary in Essex and instantly the temperature fell like the House of Usher.

I was there, on this trip organised by the RSPB, to look for these armies of pleasure-seekers, these birds that think winter on a British estuary is a soft option: birds for whom the huge skies and vast muds are a kind of Caribbean holiday, a welcome break from harder times in, for example, Siberia.

That's where the brent geese come from: rather favourites of mine, as it happens. They are not much bigger than a decent-sized duck, mostly black, but with a white necklace and a white bum: a picture of chunky monochrome elegance. One brent is smart: many brent geese are a glorious expression of dapperness, and perhaps the chief pleasure of this trip was the savouring of the joys of companionship, of species solidarity. There are birds here that have their being in numbers: the vaster the better.

There are 30,000 or so waders, ducks and geese wintering on the Stour estuary: I know, because highly numerate RSPB people count them. You can say what you like about Britain, but we've got an awful lot of coast, and birds from all over the frozen north come hammering down here for a winter on the shore. These places matter for the world. Which makes it a global disaster when we destroy an estuary: the Orwell, a few miles north, holds nothing like these numbers, because the vast muddy feeding grounds and salt marshes were demolished to make way for port construction.

But the Stour is, by comparison, untouched, and the brents gather in their dark, solemn way, sometimes making long lines across the sky, at other times engaging in noisy conversations on the ground. But there are concerns about the brents here: the observers haven't seen enough juveniles, and if this view is proved correct after the next count, it could be a serious problem. And it's all to do with lemmings.

Lemmings squeezed

If there are not enough lemmings to eat, Arctic foxes turn to brent goose chicks. Lemmings are boom-and-

bust breeders: some summers they are present in huge numbers, prompting the mythologies of mass suicide. Other summers they are comparatively scarce, and the brents are back on the menu.

In the natural cycle, these strange oscillations are sustainable. But there's a rum fact about lemmings - and rum facts about lemmings are affecting the future of British birds. Lemmings may live in an extreme environment, up in the Arctic Circle, but they don't hibernate.

Instead, they spend the winter lurking below the snow, feeding on grasses, many of which they have clipped and saved in advance. It follows, then, that they need snow: and there are suggestions that with retreating snow, lemmings are finding it harder to make a living. I was wearing seven (count them) layers of clothing as I stood on the deck of the barge, binoculars in my gloved hands. It didn't seem quite fair to worry about global warming in such circumstances: but even when you are wearing thermal long johns, global warming is an inescapable fact of life.

King knot

Even more than brent geese, knots like togetherness. When a knot rests, generally because the mud they feed on is covered by the rising ride, they stand together as close as they possibly can: not a knot but a blanket of knots, and I saw these gatherings time and again as we turned back into the wind and headed back for Mistley Quay.

The name is supposed to be a tribute to King Canute, a man almost as important in mythology as lemmings, though some suggest that it is just a representation of their call. But their scientific name is unambiguous on the matter: Calidris canutus.

Knot have the same fascination with tides as the monarch: in fact, they gather, sometimes in huge numbers, right at the edge of the rising water, apparently willing the waters to retreat to allow them back on to their lovely mud. They too are long-distance travellers, coming to Britain from Greenland and Arctic Canada. A hint of falcon in the sky and they are off: taking off in huge numbers, tight together, and somehow not bumping into each other.

This was a fine trip, with occasional treats of scarce birds on the water: long-tailed duck, Slavonian grebe, Bewick's swan. But these are just bonuses: the point of this journey was to savour the numbers: the hundreds and the thousands of birds for whom Britain means survival. It's a grim, tough option, but these birds are masters of the job, provided we let them get on with it.

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