Nature Studies by Michael McCarthy: In a city of falcons, it's worth looking up

City of royalty, city of riches; city of poverty, city of squalor. City of billionaire Russian oligarchs; city of hate-filled Islamist preachers; city of English gentlemen's clubs. City of 300 languages. City of black cabs, red buses, green parks. City of blue plaques, marking the homes of its famous inhabitants. City of endless variety. London's been called all of those.

What it's never been called, so far as I am aware, is city of falcons. Yet it is a remarkable fact that over the last decade, more than 20 pairs of peregrine falcons, the world's most streamlined and spectacular aerial killers, have taken up residence and begun breeding in the capital, some of them in its very heart. Their dashing flight is a London sight; their screams, a London sound. Their "stoop", the vertical dive on to prey which makes the bird the fastest creature on earth, touching well over 100mph, now takes place above traffic-filled streets, and shopping crowds, and hurrying commuters ignorant of the violent death in the air above them.

In the city's 2,000-year history, this is new. Peregrines, the "wandering falcons", are naturally birds of coastal cliffs and mountainous crags; they like to perch on sheer drops of a couple of hundred feet or more, and while London was a relatively low-rise city, from the Romans until 50 years ago, there was no such place (other than the medieval cathedral of St Paul).

Yet the advent of really tall buildings, from the early Sixties onwards, transformed London, from a peregrine's point of view, into an ideal habitat, with the sheer sides of skyscrapers offering the same benefits as a cliff face: a secure place to nest, and a perfect vantage point to spot prey (in London's case, feral pigeons).

At first, the birds were unable to take advantage of the change, as in the Sixties they were undergoing a severe national decline caused by organochlorine pesticides, which built up in the bodies of songbirds eating worms, then built up in the bodies of the falcons eating the songbirds. The peregrine population crashed across the country, hitting a low point of about 350 pairs in 1962.

But after the chemicals were banned their numbers began to recover steadily – there are now about 1,400 breeding pairs in Britain – and in the late 1990s keen observers began to notice them in London skies. The first birds are thought to have bred in 2001, and in the succeeding 10 years their spread has been extraordinary; they can now be seen haunting many conspicuous London landmarks with their dash and élan, such as the Palace of Westminster, Tate Modern, Battersea Power Station and the O2 Arena. The last time I walked past the Houses of Parliament two birds were wheeling and screaming around the Victoria Tower, chak-chak-chak-chak-chak!

The falcons do not always breed on these well-known buildings, using them rather as roosting or perching places; the parliamentary peregrines, for example, breed in an office block on the south side of the river, and the Tate Modern peregrines in a block of flats in the City. But they can be seen nearly every day, in aerial acrobatics which give familiar tourist attractions a new fascination, just as they can be seen now further afield, in suburbs such as Lewisham, Sutton and Fulham; and nearly every breeding pair (and there are perhaps 23 of them, perhaps more), has a group of devoted human followers who watch out for their welfare, sometimes in secret.

My own local peregrines are the Fulham ones, monitored by a French scientist long resident in London, Nathalie Mahieu; after four years in local residence, they bred successfully for the first time this summer, producing three chicks on the top ledge of a 15-storey building on the borders of Fulham and Hammersmith. I went to see them yesterday on the way to work. Nathalie pointed out to me Charlie, the female, who has been around since 2007 and who, the ring on her leg reveals, was born on a sea cliff on the coast of Sussex. ("Funny," said Nathalie, who hails from Normandy, "we both come from the Channel coast and we both ended up in west London.")

Charlie sat on the top of a rail, 200 feet up. The sun caught her slate-blue back, and the pinkish-brown flush on her barred breast, and her black hood, while everything that was terrible about her shone yellow: the pitiless eye, the hooked beak, the implacable talons. She was a dealer in death. She surveyed the world beneath her like an empress.

City of falcons, indeed. There's always been magnificence in London, alongside the poverty and the hubbub of everyday life; now there is even more.

Birdwatching onthe terraces

The peregrine is the species I hope to add next to the list of birds, currently 13-strong, I have seen from Craven Cottage, the home of Fulham FC, unique among Premier League football grounds in that you can birdwatch from its terrace overlooking the Thames. I was at the Cottage last weekend and scanned the skies for peregrines in vain, although I did encounter an unforgettable sight: the final scoreboard saying Fulham 6, Queen's Park Rangers 0.