Peter Ross: Ornithology becomes a matter of life and death for the most passionate bird lovers

• Andrew Knowles-Brown with Gorby the golden eagle

Almost three million of us go birdwatching; within that group are the hardcore twitchers who will travel from one end of the country to another on the promise of a glimpse of a rarity. In Scotland, birds lay nest eggs of the financial sort; the Mull of Galloway bird reserve, for example, generates more than 100,000 each year for the local economy.

It feels a little vulgar, though, to talk about the monetary benefits of birds. They are above all that. There's something about them that gets into the soul. It is perhaps, as suggested in the BBC 4 series Birds Britannia which begins on Wednesday night, that the vast majority of us live in towns and cities, and birds remind us in an enjoyably elegiac way of the countryside from whence, pre-Industrial Revolution, we came. As George Orwell once enthused in austere, blitzed, post-war London, "I have seen a kestrel flying over the Deptford gasworks, and I have heard a first-rate performance by a blackbird in the Euston Road."

I love birds, in part, for the music of their names; the way they sound and the way they can be made to flock together on the page. Corncrake and kittiwake, fieldfare and chiffchaff, jackdaw and jay. I love, too, seeing them in just the right place. A tai chi heron in a burn at twilight. An aerial display team of swallows skimming the grass in an English churchyard. A robin - stained red with the blood of Christ, or so the story goes - bobbing for breadcrumbs in the snow. I even have a soft spot for those birds which are, in general, unpopular with the public. I adore the thuggish magpie, the colour of a mobster's spats, and I admire those resourceful herring gulls which shriek in triumph on stealing your seaside chips.

Seeing birds in their proper context is important. They are part of the landscape and therefore part of who we are. The ornithologist James Fisher published his bestselling Watching Birds in 1940, a historic moment when those Britons watching the skies were more alert to the darkling shadow of a Stuka than the friendly silhouette of a starling. Yet this, for Fisher, was the very point. "Some people," he wrote, "might consider an apology necessary for a book about birds at a time when Britain is fighting for its own and many other lives. I make no such apology. Birds are part of the heritage we are fighting for."

Birds can still inspire deep passion. Some people seem enraptured by a particular family or species, so that their lives become intertwined with those of the birds. A husband and wife in Cupar have been ringing and studying mute swans on Duddingston Loch and elsewhere since 1978.A man in Dumfriesshire has been counting rooks for more than 50 years. There is, however, one individual who, for me, exemplifies the British obsession with birds. On an overcast Thursday morning, I go to meet him.

Andrew Knowles-Brown is 55 years old and lives on Crookedstane farm, near Elvanfoot in South Lanarkshire, amid the beautiful, desolate uplands through which the fledgling Clyde flows. This is a good place to have your home when you are Europe's most accomplished breeder of eagles - 1,000 hilly acres hoaching with hares.

Knowles-Brown is tall with steel-grey hair and a controlled, gentle manner. His Barbour jacket and left hand bear the marks of his beloved birds of prey - the former ripped in places, the latter having suffered a deep scratch just above the wrist. Once, a golden eagle pierced him through the palm, the talon emerging between two of his fingers. He keeps antibiotics handy in order to fight off blood poisoning.

In his kitchen, warmed by an Aga and overlooked by a stuffed sparrowhawk and barn owl, Knowles-Brown says: "My whole life has revolved around birds." He has been a keen falconer since the age of nine when he lived in Buckinghamshire. Back then he had been having Sunday lunch when he looked out the window and saw a man crossing a field, swinging something - which he later understood to be a lure - round his head. When the man came back he had a falcon with him. "I have a terrible memory, but I can still remember looking up and seeing this bird eating on John's fist. That stays vividly in my mind."

John White, a local printer and trade unionist in his early thirties, was to become Knowles-Brown's mentor. He taught him falconry. At that time, in the mid-Sixties, there were only a few hundred falconers in the UK; now, an estimated 25,000 people keep hawks.

As the sport has soared in popularity, so Knowles-Brown's expertise has grown. While living in England, he flew goshawks, but having relocated to Scotland, he switched to eagles. His first, a golden eagle called Gorby - after Mikhail Gorbachev; it came to him from Moscow Zoo in late 1989 - is still with him and remains a favourite. How many eagles does he have now? "I admit to 25. But I don't like to count them. It frightens me if I do. Far too many, really."

His birds are kept in a number of purpose-built barns and pens, off limits to everyone but Knowles-Brown and, on occasion, his wife, Jacky. He bred his first eagles in 1996. Five years later, working with a scientist from the University of Abertay, he was the first person in the world to breed an eagle using frozen sperm, opening up the possibility of breeding to safeguard endangered species. He now breeds several different types of eagle, using both natural methods and artificial insemination.

The breeding season lasts from February to July, during which period Knowles-Brown cannot travel more than half an hour from Crookedstane in case his incubators break down.Between 15 and 20 birds hatch here each year. He keeps the firstborn of each new species and sells or lends the rest to people he believes will do a good job of looking after them. His price per bird ranges from around 3,000 to more than 15,000 depending on species.

Eagles are, he admits, an obsession. "They are my life." But what is it about these particular birds which so consumes him? He finds it hard to say, umms and ahhs, and talks about an innate ability to "read the birds".

To be a good falconer and breeder of hawks appears to require a degree of empathy, so perhaps his passion for eagles is to do with a feeling of close identification with a creature in which murderous ferocity is a prized quality; you don't get to access those sort of feelings with a pet budgie.

"I think it's probably also to do with the adrenaline rush," he says. "A lot of people are intimidated by eagles, especially when they are at their flying weight. They can put the fear into you, because they are physically able to do you a lot of damage. So you do have to very careful and treat them with respect as they are basically wild."

After lunch, we go hunting with one of Knowles-Brown's favourite birds, an African crowned eagle named Isis, born here two years ago. She looks extraordinary, a queen in exile, regal and dreadful; a crest of feathers above disdainful brown eyes on either side of a hooked black beak. Her plumage is white, brown and black. The talons, curved like scimitars, are huge. They are meant for plucking monkeys from trees, but in South Lanarkshire the eagle must content herself with rabbits and hares.

Knowles-Brown carries Isis on his right hand, protecting his flesh with a heavy leather glove. Round her legs are jesses - leather straps connected to a nylon leash. The falconer places a hood over her head and we walk out to hunt. It's strange to see this obviously foreign bird in a landscape of brown hills and old Scots pines. Strange, too, to see such an ancient sport practised while, close by, lorries speed along the M74 and Virgin trains shuttle between Glasgow and London.

Falconry is one of the oldest sports in the world, with its origins approximately 3,000 years ago in central Asia. It is thought to have taken place in Britain for more than 1,000 years. Falconry goes back so far in our culture that some of its terminology has entered common parlance. A "haggard", for instance, is an old hawk. A "mews" is where the hawks are kept. A cadge is the wooden frame on which, traditionally, birds were carried by servants during hunts; the slangy verb "to cadge" is supposed to come from falconers asking for a particular hawk.

Knowles-Brown removes his eagle's hood and walks with her across the marshy ground. Both are alert to the movements of Pilot, a black pointer, which noses through the rushes, trying to flush out prey. Suddenly, the dog stops, head down. It has found something. A rabbit darts from cover.Knowles-Brown releases Isis. We see her five-foot wingspan; her talons tilted for impact. She strikes. The rabbit squeals once and is silent. The life is quickly squeezed out of it. Gripping her prize, the eagle hunches over protectively in a movement known as "mantling". When she looks up, her beak is crimson. Knowles-Brown allows her to eat the heart. "There's a good girl," he says.

It feels like a long way from a blue tit on a bird table to this gore-steeped exotic monster on a moor, yet both are beneficiaries of the British love of birds. Exactly why we feel so fervent is difficult to explain. Perhaps, as Isis would appreciate, it's just something in our blood.