Garry Bagnell is cruising down a country road in Great Yarmouth, England, when his beeper lights up with a bulletin. A shorelark – a distinctive bird with yellow and black markings – took a wrong turn somewhere over Norway and is getting its bearings on a beach an hour's drive north. Time to step on the gas.

"I need that bird, I need it," said Bagnell, a 46-year-old accountant and hardcore practitioner of British twitching, or extreme – and extremely competitive – birdwatching. "When a bird you haven't seen drops, you've got to chase it. That's going to bring me up to 300 [different species] spotted for the year. You don't understand how competitive this is. For some people, this is life and death."

In other countries, the world of birdwatching may be a largely gentle place ruled by calm, binocular-toting souls who patiently wait for their reward. But in Britain, it can be a truly savage domain, a nest of intrigue, fierce rivalries and legal disputes. Fluttering somewhere between sport and passion, it can leave in its path a grim tableau of ruined marriages, traffic chaos and pride, both wounded and stoked.

This is the wild, wild world of British twitching.

Britain isn't the only place that has hatched a culture of fierce birdwatching. In the United States, book-turned-Hollywood-film The Big Year chronicled the quest of three men vying in long-held American competitions to spot the most species in a single year. Nevertheless, observers say the intensity of the rivalries and the small size of the twitching community – in the thousands – have singled out British birders as some of the world's most relentless.

One of the fiercest rivalries, for instance, involves Bagnell's former mentor, now nemesis: bird guide Lee Evans, 53. Evans, who dubs himself the "judge, jury and executioner" of birdwatching in Britain and keeps his own twitcher rankings, has long been challenged by 41-year-old grocer Adrian Webb.

To take on the master, Webb took 12 months off from work in 2000, spending $22,000 and driving 140,000km to break Evans's record of 386 species of birds seen on the British Isles in one year. They trash-talk on the birding circuit like prizefighters.

Webb is known to drop his grocer's apron to chase a rare bird and claims to have broken Evans's record in 2000. Evans – a polarising figure on the birding circuit – does not recognise Webb's claim to the title.

"In America, birdwatching is still mostly a pastime," said Evans, who is on his fourth marriage and blames his divorces partly on his obsession with twitching. "But in Britain, birdwatching can be bitter. It can be real nasty business."

Evans also insists he has been the victim of underhand tricks, citing an incident when he was racing to see a rare bird in Scotland. He had lined up a plane to take him to a sighting on a remote island, only to find that a group of rival birders had stuffed the palm of his pilot "with a few extra quid" to take them instead.

A term coined in the 1960s to describe the jaw-rattling sound of chasing after rare birds on rumbling motorbikes, "twitchers" are narrowly defined as bird-watchers willing to drop everything to chase a sighting. More broadly, it includes those who see a bird within a few days of an urgent bulletin.

These bulletins are typically sent out by services such as the Rare Bird Alert, which obtains its information in real time from a vast network of birdwatchers across Britain. Once notified of a sighting, the service issues urgent messages to its 21,000 subscribers via pay-by-the-month pagers and smartphone apps.

In one of dozens of similar scenes of "twitcher madness" in Britain, police were forced to cordon off streets after hundreds of desperate birdwatchers descended on a suburban home in Hampshire in 2012 to see a rare Spanish sparrow.

Garry Bagnell looks for a shorelark at Great Yarmouth. Unsuccessful sightings are known as 'dips'. Photograph: Andrew Testa/The Washington Post

For a mostly male sport with an average age over 50, however, twitching can also tempt fate. Last October, a top British twitcher, Tim Lawman, had a heart attack while on the trail of a Radde's warbler in Hampshire. "It was a new bird for him, and in all the excitement of rushing to see it, he just keeled over and died," Evans said.

A smartphone app to help British birders is being advertised as an essential tool when "there have even been recent cases of violent clashes between bird watchers as people desperately try to get the very best spots".

In 2009, Bagnell said, he and other twitchers were aghast when two elderly rivals on the circuit went for each other's throats. "One was saying he'd seen a bird, and the other said he didn't believe him," Bagnell said.

Though most twitchers are bird-lovers, the sport is mostly about the chase. Bagnell, for instance, drove 90 minutes and searched the ground for a half-hour before he spotted the coy shorelark in beach scrub. He eyed it for a few moments before tweeting his find, then moved on. "I've got another bird to get three hours away," he said.

The most unfortunate twitchers race many kilometres to spot a bird only to find that their flighty subjects have flown off – a bummer known in the twitching world as a "dip". One of the most infamous dips came as Webb pursued a long-tailed shrike in the Outer Hebrides off mainland Scotland. The boat he and 12 others had hired died in choppy waters, forcing a daring rescue by Her Majesty's Coastguard. "We were worried for our lives for a bit, but we were more worried about not seeing this bird," he said.

Within the world of twitching, there are countless rankings – lifetime lists, annual lists, semiofficial lists, slightly more official lists – which are predicated on evidence. When you saw that velvet scoter wading in Wales, were there witnesses? Photographs? If not, claims all come down to trust.

Many see twitching as an outcrop of the British fascination with "spotting" things – most notoriously, trainspotting, a hobby that involves the obsessive pursuit of seeing as many locomotives with your own eyes as humanly possible. But others say it may simply be a case of boys who refuse to grow up.

"Years ago, British boys used to spend their childhoods collecting birds' eggs – now you wouldn't dream of doing such a thing," said Brian Egan, manager of the Rare Bird Alert. "But what they can do as adults is chase sightings of rare birds. So that's what they do."

This article appeared in Guardian Weekly, which incorporates material from the Washington Post