Their presence astounds me: it is their absence that should have astounded me all those years ago. They look wildly incongruous: the truth is that they are completely congruous. Still, a pterodactyl in London: it's not what you expect.

Well, they look like pterodactyls: weirdly, impossibly, big, and flying with a wingflap so slow and measured that it looks as if they should fall out of the sky. There is something grim and threatening about them as they come in low over the trees: something deeply atavistic in the grave panic of their landings. They are certainly too big to hang about in trees.

I counted a dozen or so nests: huge, untidy rafts of stick floating in the high branches. It's all just beginning again: the Kew heronry is back in action, easily visible from the towpath across the Thames. They are solitary creatures, herons, usually seen as single birds at the edge of a river or a lake: grey, brooding, still, with that spear of a beak perpetually at the ready.

But at breeding time they are overcome with sociability, and they come together in numbers, quarrelling, getting in each other's way, balancing uneasily on impossibly slender twigs and barking. Having established such a place, they like to come back every year: when autumn strikes, the trees are revealed full of nests supported by a lavishly whitewashed trunk.

Cleaner waters and milder winters - hard to fish in an iced-up lake - have brought the numbers of London herons soaring. I hardly ever saw a London heron in the 1960s; the Kew heronry started up in 1989. It was a false start, but they came back again, and these days, there tends to be a dozen pairs most years.

There are likely to be 300 pairs of herons nesting within the boundary of Greater London, there were only 100 in 1985. For a heron, the heart of metropolitan London is Walthamstow; there is a heronry of 100 pairs at the reservoir. Some will already be on eggs, stealing a march on everyone else.

Herons are traditionally flighty birds, taking to the air with a great bark when a human gets within 100 yards, indicative of centuries of persecution. But their culture is changing. You can see herons fishing solemnly at places like Barnes Pond, while children alongside throw bread at ducks, dogs defecate and cyclists whiz past. Timorousness was once a survival ploy: now boldness pays.

Water plus sun equals life: the most important equation you will ever read. It makes sense, then, to look after the water. Herons, top of the food chain, indicate that all is well with the watery environment. But the price of conservation is constant vigilance: a conservationist, like a Forth Bridge painter, can never pride himself on a job well done.

This week the Government released its Water Strategy, to a chorus of sniffs from people who know what they are talking about. The RSPB said that we would struggle to keep up with growing problems of flooding, drought and pollution: that it was all too much consultation and not enough action.

The problem is that under the warped priorities of governments, a thing like water policy is seen as drab, unsexy and really not worth making a fuss about. It's only the stuff of life, that's all. We've got the herons back: we need to keep them. For our own sake.

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