Listen carefully and you can still hear the nightingale's song

Five levels of sound are layered on top of one another. The hissing is the traffic on the M25, London's orbital motorway. The growl is the jets, taking off from Heathrow airport. The rumble is the railway heading for Waterloo. The faint racket is a party at a nearby hotel. And then there is the song; that's the nightingales.

Clear-cut, liquid, astonishingly loud, it pierces through everything else as we stand, spellbound, in the dark. Once, everybody is southern England knew this magical sound, for the six weeks when it is audible in April and May, but now, for most people, it has long been left behind, swamped by the din of modern life.

Three birds are, echoing each other across the common, the nearest less than 20 yards away but invisible in its clump of scrub. They pour out phrase after phrase, long yearning notes and melodious low whistles interspersed with rattles like gunfire, and they go on and on, their tirelessness as extraordinary as the song itself.

Hearing it in the darkness, you understand at once why this is the most celebrated bird in literature, from Chaucer onwards. Shakespeare is full of nightingales, and Keats wrote the most famous ode in English after listening to one pour out its soul in his fiancée's Hampstead garden. But for most of us now, that's the only place we can encounter these little brown birds with unforgettable voices: in literature.

It's partly that we are an urbanised society, living far from the countryside and overwhelming the night with artificial light and noise. But it's also because the birds are gradually disappearing, shrinking in range back to the south-east corner of England.

Once they occurred as far north as Cheshire and as far west as Devon; now they are mostly contained south-east of a line from Norfolk to Hampshire. No more nightingales breed in Greater London, and there never was a nightingale that sang in Berkeley Square (if anything, that was a robin, for robins also sing at night - especially near street lights.)

However, there are nightingales just beyond the M25; and as I did not want my 14-year-old daughter to grow up without knowing what a nightingale sounded like, I asked her if she would like to go on a nightingale safari. She said she would.

On Saturday evening, with a couple of her friends, we found ourselves on Bookham Commons, a jewel of a nature reserve run by the National Trust near Leatherhead in Surrey.

Careful management has produced a habitat of scrub-rich grassland that nightingales find congenial: at least five pairs are present. They begin singing at dusk, hesitantly at first, distinguishable from the blackbirds and song thrushes by their loudness; but when we returned after dark they were singing alone. "I can hear them in my house half a mile away with the windows shut, the curtains closed and the TV on," said Ian Swinney, the warden.

We stood for an hour, the three teenagers captivated by the birdsong, fluting, whistling, and warbling, phrase after phrase hanging on the night air, dominating the noises of traffic and party.

There may not be any in Berkeley Square, but just 20 miles away, nightingales are alive and well and singing their hearts out.