One snowy dawn in March, I went hunting for squirrels in the Lake District. In the silent and empty woods beneath the Aira Force waterfall, the only thing moving was a solitary red squirrel, balanced on a nut-filled feeder hanging from a tree. If you grew up, as I did, with the grey squirrel, seeing a red squirrel is a shock. We’re used to the grey – a sleek, North American import, swaggering across parks, raiding bird tables, all fat haunches and bulbous black eyes. In contrast, the red squirrel, although native to Britain, looks exotic: so dainty and alertly pretty, with fine tufts of hair above its ears as extravagant as the eyebrows of Denis Healey. Here, in the snow, this forest sprite quivered with improbable, balletic grace and then – clang – slipped on the icy lid of the feeder and fell to the ground. It landed on its feet.

Julie Bailey, a former gymnast with a cascade of red hair, had picked me up from the nearby town of Penrith and driven her black 4x4 along slushy roads to admire this natural acrobat. At Aira Force, she stepped out of the car and, leaning on a stick, walked carefully across the snow. She and her husband, Phil, used to enjoy watching red squirrels at their feeders in the garden; these animals were still a common sight across northern Cumbria a decade or so ago. Bailey worked in pharmaceuticals and coached boys in gymnastics, including her son. But in 2005, she broke her back. She couldn’t walk for four years. Seventeen spinal operations later, she only walks thanks to a spinal cord stimulator, powered by a battery in her stomach. When it malfunctions, she collapses. She doesn’t make a fuss, but she is in pain 24 hours a day and is intolerant to painkillers. “Because I was stuck at home,” she said, “I started taking more notice of my squirrels. They really gave me a purpose.”

During the Christmas holidays of 2009, she was startled to see a stranger in her garden: a grey squirrel. Over the next three weeks, her reds became sick, their faces swelling with terrible sores before they dropped dead. “It was absolutely devastating,” said Bailey.

Bailey joined a band of concerned Cumbrians – accountants, police officers, cleaners, carers, construction workers, retirees – who spend their spare time working to stop red squirrels in their county slipping towards extinction.

Red squirrel conservation is not like most other efforts to save wildlife. It consists of exterminating one rival species: the grey squirrel. A few weeks after the death of her red squirrels, Bailey began to fight back on the side of the reds. She set up strategically placed feeders so that she could pull up quietly in her car, wind down her window and shoot the greys with an air rifle. Her husband got a gun and joined in. The first time I spoke to Bailey by telephone, I asked her how many squirrels she and Phil had eliminated since they began. She paused. I could hear the clicking of a mouse through a spreadsheet. “Four hundred and sixty nine,” she replied.

When I visited her home a month later, I found a shrine to the red squirrel. The time was told by a red squirrel clock, the woodburner was adorned with cast-iron squirrels, Bailey’s study walls and carpet were squirrel-red; there were ornamental squirrels made by a local sculptor, a red squirrel jigsaw, goblet, boot brush, paperweight and piggybank. We drank tea from red squirrel mugs, and sat by a grey-coloured freezer. When I asked what was inside, Bailey opened the door and pulled out neatly butchered hunks of grey squirrel. “All our greys go in that freezer and we eat them. Not everyone’s cup of tea, but that’s what we do. Very healthy meat. Phil loves his squirrel curry, because he just loves curry. I love it in a stew, so it falls off the bone like pulled pork.” Bailey was collecting the pelts to make a grey waistcoat.

Britain’s red squirrel is distinctive for the colour of its fur and its tufted ears. Photograph: Steward Ellet/Natural England/PA

Britain, Ireland and Italy are the only countries in the world inhabited by both red and grey squirrels. In Britain, the greys, which were introduced in the late 19th century from North America, appear to be exterminating the reds. Britain’s 140,000 red squirrels have been pushed to the margins – islands, such as the Isle of Wight; northern Scotland, parts of Cumbria and Northumberland – as 2.5 million greys have taken over.

An epoch of human globalisation is mixing up species like never before. It has also created a new academic field, invasion biology, which examines how some non-native animals and plants (such as rats and Japanese knotweed) wreak havoc in new settings, spreading disease or out-competing native flora and fauna. Some consider the grey squirrel one of these “alien” invaders imperilling the red. But is its slaughter a futile expression of nativist xenophobia? And is it ever ethical to target one species for destruction, in order to conserve another?

Many mainstream conservation charities have, quietly, decided that a cull is an acceptable solution. There will soon be thousands of volunteers working to kill grey squirrels in England, Wales and Northern Ireland. (Scotland has its own, rather successful anti-grey campaign.) This army is being assembled by Red Squirrels United, a new campaign supported by more than 30 conservation groups and £3m from the Heritage Lottery Fund and the EU Life programme, the European Union’s funding body for environmental projects.

Red Squirrels United is the largest programme to eliminate an invasive species in Europe. It is also the most controversial. More than 95,000 people have signed an anti-cull petition. Their comments reveal an abhorrence for this kind of conservation: “This plan is barbaric”; “My son and I love watching the grey squirrels play where we live”; “Humans do not OWN animals’ lives”; “These creatures are here to stay and part of our countryside now. You cannot turn the clock back.” As these remarks show, the conflict between two very different species of squirrels is also a dispute between their very different kinds of human champions. How this battle plays out – reds v greys; conservationists v animal rights activists; northerners v southerners – could eventually shape the fate of other non-native species around the globe.

The red squirrel, Sciurus vulgaris, is a common animal across much of Eurasia. Sciurus carolinensis, the grey squirrel, is one of more than 2,800 non-native species in Britain. Like the signal crayfish, a North American creature that has been outcompeting the native white-clawed crayfish since it was introduced in 1975, or the Asian hornet, a hefty wasp that turned up in Gloucestershire last summer and devours honeybees, the grey squirrel is classified as an IAS: an invasive alien species. The International Union for Conservation of Nature puts it in the top 100 most harmful invasive species in the world. It strips and eats bark, damaging and sometimes killing young trees, which can prevent the production of high-quality timber. Grey squirrels are also accused of preying on birds’ nests (although scientific evidence of greys causing declines in bird species is, so far, scant). Most emotively, they appear to be consigning the red squirrel to oblivion.

In 1876, Victorian landowners shipped the first grey squirrels from North America and released them into English parkland as amusing ornaments alongside peacocks, muntjac deer and other exotic status symbols from distant lands. When squirrels spread, at first people welcomed this “sociable, easily tamed animal”, as the 1912 Manchester Guardian’s country diary put it. In 1916, another country diary approvingly noted: “It appears to be more ready to make friends than our British squirrel, but possibly it has not the same hereditary recollection of stone-throwing boys.”

As the stone-throwing boys suggest, the native red has not always been adored. In the early 19th century, 20,000 red squirrels were sold to London meat markets each year. Later, reds were widely culled by foresters because they too stripped bark in new conifer plantations. In 1903, the year the red squirrel was immortalised by Beatrix Potter as plucky Squirrel Nutkin – a defiant representative of the working class, according to one critic’s interpretation – the Highland Squirrel Club was formed to protect timber plantations from damage. Club members slaughtered 85,000 reds over the next three decades. Where reds were driven to extinction, landowners and conservationists shipped in new individuals from Scandinavia.

A grey squirrel in Regents Park, London. Photograph: Cathal McNaughton/PA Archive/PA Photos

In the 1920s, people noticed that greys were increasing at the expense of reds. In 1932, it was made illegal to release a grey squirrel in Britain. It still is: if you inadvertently trap a grey squirrel in your garden shed, you’re breaking the law when you free it. The first of many campaigns to eradicate the grey was launched in the 1930s, and the Ministry of Agriculture issued free shotgun cartridges to squirrel shooters for many years. In the 1970s, greys were poisoned by the Forestry Commission with warfarin, commonly used to kill rats.

For the last eight years, Prince Charles, as patron of the Red Squirrel Survival Trust, has promoted campaigns to exterminate the greys. He grew up enjoying the reds on his mother’s Sandringham estate in Norfolk and still feeds them at Balmoral in Scotland. “My great ambition is to have one in the house,” he once said. “Sitting on the breakfast table and on my shoulder.”

Despite royal intervention, all attempts to eradicate the grey on a national level have failed because, for many decades, the precise reason for the reds’ decline remained a mystery. Ecologists agreed that the larger greys were outcompeting reds. Greys were more willing to forage on the ground, better able to digest acorns, and would consume unripe hazelnuts, whereas reds had to wait until they were ripe – so greys were beating reds to sources of food. This theory was not wrong, but it did not explain the often dramatic disappearance of reds after the arrival of greys.

For years, people had noticed that reds were succumbing to an aggressive viral disease that caused swollen ulcers to break out on their faces. These lesions prevented the squirrel from feeding and killed it within a week. Greys were untroubled by this virus. Scientists finally uncovered the full story in the early 1990s, when new blood-testing technology revealed the role of greys as a carrier of squirrelpox. The test found that 60-70% of greys carried the infection but did not succumb to it. No British grey has ever been found dead from the virus, but every red squirrel that has ever contracted it has died. It is a pattern repeated around the world when non-native animals introduce new diseases to “naive” populations.

Greys are well adapted to squirrelpox, say scientists, because the squirrel and the virus evolved together for centuries in North America. One study has found squirrelpox antibodies in America’s greys – a sign that they have had the infection but have fought it off. In Britain, the disease decisively shifts the balance of power from defenceless red to resistant grey: scientific modelling predicts that where the virus is present, greys replace reds up to 25 times faster than where it is not present. Prof Julian Chantrey, a veterinary pathologist at the University of Liverpool, has studied an isolated red squirrel population at Formby on Merseyside, which has been hit by squirrelpox brought in by arriving greys. He believes that the reds’ extinction is inevitable if greys are not kept away. “We’ve seen it happen across southern Britain. Where the habitat hasn’t been kept grey-free, invading greys will spread the virus to any recovering red population and it will crash again. The population gets to a lower and lower level, and then they are wiped out.”

Two decades ago, it was predicted that reds would be extinct in Cumbria by now, but they are not. Red squirrels may be defenceless against squirrelpox, but their cuteness has given them a staunch ally: humans. “I don’t think there would be any squirrels left around Cumbria if it wasn’t for Julie Bailey,” said Andrew Hodgkinson, a softly spoken, earnest conservationist who has worked in the county as a red squirrel ranger, monitoring reds and killing greys, for two years.

Bailey began volunteering for Penrith & District Red Squirrel Group after her accident. Because her back pain caused her to be awake for much of the night, she worked all hours and soon became administrator, treasurer and trustee. She also supported some of the several dozen local squirrel groups across northern England. If the accident hadn’t happened, said Bailey, she wouldn’t be “making the difference I think I’m making for red squirrels, so some good has come out of it”.

Red squirrel conservation attracts lots of grassroots groups (such as Bailey’s) that support a confusing array of national campaigns, as I learned at The Red Squirrels United Knowledge Fair, which was held in an upmarket Belfast hotel in March. Red Squirrels United is a three-year project supported by groups including Prince Charles’s Red Squirrel Survival Trust, the European Squirrel Initiative and the UK Squirrel Accord. The latter is a mix of government agencies and charities that is developing an oral contraceptive for grey squirrels.

Beside a banner declaring “I’m nuts about reds!”, a series of scientists and conservationists gave PowerPoint presentations to describe their plans to halt the march of the greys, featuring military-style maps of marching grey dots, and punctuated with battle metaphors (defending red “strongholds”). The audience was comprised of passionate volunteers, gentle, fleece-wearing charity workers and tweedy representatives of big landowners, who hate the grey squirrel because of the damage it does to their valuable plantations. “It’s absolutely wonderful to come here and talk about controlling greys,” announced Andrew Kendal of the European Squirrel Initiative, which is funded by landowners and lobbies the EU. “Fifteen years ago, it was virtually taboo.” I learned about growing propaganda efforts on behalf of the red squirrel – Ulster Wildlife Trust is developing a new children’s book called The Greedy Grey – and discovered innovative despatch devices, such as a new trap that automatically kills the grey squirrel caught inside.

A red squirrel darting along in Kielder Forest, Northumberland. Photograph: Owen Humphreys/PA

Conventional traps catch the grey squirrel alive and rely on someone then either shooting or hitting it over the head. Opponents describe this as “bludgeoning” them to death, but the Wildlife Trusts prefers to call it “cranial despatch”. The approved, legal method is one firm “tap” on the head with a stick. Nationally, conservation charities such as the Wildlife Trusts and the National Trust are coy about their role in controlling greys. They lose members when they speak in support of killing the non-native squirrel. But in areas where there are still reds, such charities often facilitate culling. Once red squirrel conservationists have the support of landowners, the key, according to Julie Bailey, is “boots on the ground”: both volunteers and professionals willing to devote their days to killing greys.

Private landowners are often reluctant to allow conservation volunteers on to their land, especially if they are wielding traps or shotguns. They prefer professionals, so Bailey’s Penrith & District Red Squirrel Group must raise £100,000 each year for three full-time and two part-time squirrel rangers. “We’re struggling, if we’re honest,” she said. “We don’t get a penny from the government – we’re voluntarily led and self-funded.” But Bailey’s group covers a quarter of Cumbria, and she has gathered some impressive data. In 2014, her group recorded 2,702 sightings of reds and killed 2,224 greys. In 2016, there were 3,306 red sightings and 1,806 grey kills. Fewer grey kills indicate that the cullers are winning.

New technology is helping. Last year, 69% of the greys killed in Penrith were shot; the rest were trapped. Shooting greys has been revolutionised by thermal imaging cameras. These expensive devices, which reveal the warm bodies of live animals in dense woodland, make it much quicker for shooters to locate greys. In January this year, 74 of 143 grey kills by the Penrith rangers were assisted by these cameras. Personally, Bailey prefers shooting to trapping. “I don’t do cranial despatch, it’s just too close for me,” she said. Not that shooting is easy. “It’s bloody awful. Because I shoot grey squirrels, people think I’m a heartless, murdering cow. It hurts to kill something, but if I want to keep my red squirrels, I want to do it.”

“When I look at a grey, I see a rodent. When I look at a red, I don’t, even though they are the same thing. It’s the cuteness factor. The tuft on the ears and the native/non-native gets me every time – they should be here and the greys shouldn’t. I’ve got no problem with greys in North America. I certainly wouldn’t support culling them in their country. But here we can’t have both living side-by-side. We’ve got to make a choice and undo what the Victorians did.”

During my two days with Bailey in the red squirrel territory of Cumbria, I asked every person I encountered about squirrels and could not find a single supporter of the greys. Down south, where the only wild squirrel anyone under 50 remembers is the grey, there is much more hostility to the idea of killing them.

There haven’t been red squirrels around the Tonbridge, Kent headquarters of Animal Aid since the 1960s. Here I met John Bryant, a courtly, white-haired man who is a pioneer of humane pest control. He’s a consultant for the charity, which has been drumming up opposition to the grey cull, and fears this hostility to a non-native animal is a product of xenophobic times.

Bryant’s earliest memory is of trying to get a closer look at a crow on the council estate where he was born. “It flew away, quite rightly, because my species is an enemy of that bird”. He became a helicopter engineer, played football for Yeovil Town, and joined the RSPCA to oppose fox hunting. He learned about slaughterhouses and vivisection labs, and later ran an animal sanctuary. He became vegan, but doesn’t want to be holier-than-thou because so many things that vegans depend upon – from cardboard boxes to roads – contain animal products.

Bryant’s experience as a humane pest controller has taught him the futility of killing species to “control” their numbers. Kill one urban fox and another one quickly takes over the territory. Rat poison is a food, which attracts more rats. Rather than trapping grey squirrels, Bryant removes them by putting a cat-scented cloth in their drey. The mother will then take the babies somewhere else.

The new drive to cull greys is backward, said Bryant. “It’s just, ‘Kill them.’ The same old thing. It won’t work.” Bryant remembers the 1950s cull, when the police paid schoolchildren a shilling for grey tails. He heard a story that, after depositing a tail, kids would go round the back of the station and the officer would sell it back to them for 6p. “That’s what human beings do – ‘There’s a little racket we can get into.’”

Bryant identifies many hypocrisies in killing greys. The red is not globally endangered – it’s common across Northern Europe and Siberia – and argues that the native/invasive distinction is nonsensical when so many reds were reintroduced from Scandinavia. For Bryant, the worst aspect is volunteers being taught to kill the greys by hitting them. “Clearly, that’s a horrific way to die,” he said. Most animals we hunt are also spared during their breeding season, but greys, like rats, can be killed all year round, and so lactating females will die, leaving babies to starve to death.

Squirrel ornaments in the home of Julie Bailey. Photograph: Christopher Thomond/The Guardian

Bryant is unmoved by the scientific consensus that the red squirrel will become extinct in Britain if we don’t control the greys. “I don’t think that is the case, but so what? It’s a squirrel,” Bryant replied. “Does it matter what colour it is? They are the same thing. The damage they do – which is very little compared with the damage we do – is strip bark and take eggs. That’s what squirrels do, whatever colour they are. It’s just this ‘invasive species’ label, where what I call biodiversity geeks say, ‘It’s all got to be native.’ I don’t go along with it.”

Bryant cites two environmentalists who argue against the prevailing view that we should kill the grey to save the red. Stephen Harris, professor of environmental sciences at Bristol University, now retired, wrote 10 years ago that we should accept that the grey squirrel is here to stay, and that the best place for the reds was small islands. More recently, the environmental journalist Fred Pearce has argued that ecosystems are always changing and invasive species should be celebrated. The vast majority of Britain’s flora and fauna arrived in the last 10,000 years. Nothing is “native” – everything is visiting. For Pearce, the alleged damage caused by most “invasive” species, such as Japanese knotweed, is overstated by grant-seeking bureaucrats and sensationalising media. The government claims knotweed costs the British economy £170m a year, but Pearce calculates it as far less. The Environment Agency calls it “indisputably the UK’s most aggressive, destructive and invasive plant”, but only spends £2m each year on knotweed eradication. Pearce also notes that, in 2009, the racist BNP branded the North American signal crayfish “the Mike Tyson of crayfish … a diseased, psychotic, evil, illegal immigrant colonist [that] totally devastates the indigenous environment”.

I asked Bryant why he thinks wildlife lovers and scientists are killing grey squirrels. “It’s this immigration thing. It looks as if the whole of Europe is turning into a barricaded society. ‘We don’t mind people as long as they are our people. We don’t like these foreign squirrels coming in and taking over.’ It’s intolerance, and it’s illogical.”

Opponents of culling grey squirrels believe there must be a way to save the red without killing greys. There are three technological solutions, but it is unlikely any of them will work quickly or thoroughly enough to deter volunteers from their efforts at grey squirrel eradication. One is a vaccine to save reds from squirrelpox. Dr Colin McInnes, a scientist at the Moredun Research Institute in Scotland, developed a prototype vaccine. “It gave us hope that we were on the right lines, but unfortunately we have no funding at the moment,” McInnes told me. Vaccine research is not cheap. If granted further funding, scientists must develop a method of delivery, such as an oral bait, and trial and licence the vaccine. McInnes doubts that a vaccine would ever be cheap enough to save reds across the land. “We always envisioned that the vaccine would probably be used to protect specific vulnerable [red] populations that we could trap and inoculate,” he said.

Another option is a contraceptive for greys, which the government is currently funding. But a workable formula is at least a decade away. A contraceptive could never eradicate grey populations and nor could it be used in territory shared by reds and greys – such as Cumbria – for fear of accidentally sterilising the reds.

There is more excitement about a third fix – a “biological” control. The pine marten is a predatory member of the weasel family, which is also a native species. Research by Emma Sheehy in Ireland has linked a surprising grey squirrel population crash to the pine marten’s resurgence. Ecologists think the pine marten may create a “landscape of fear” for the greys. They also believe that because the reds (which have evolved alongside the marten for centuries) weigh less than greys, they will be more easily able to escape by running to the ends of tree branches, where they cannot be reached. Dr Sheehy is studying the impact of martens on red and grey squirrels in Scotland, but says there is no guarantee that the impact she observed in Ireland will be replicated there, as the pine marten has more alternative prey to the grey squirrel, such as field voles. So she believes that suggesting that pine martens could solve Britain’s squirrel war is overstating the case? “I do,” she said. “Pine martens certainly could potentially be a tool in red squirrel conservation but research is still under way.” It’s also true that most owners of shooting estates won’t tolerate the pine marten devouring pheasant chicks and eggs in their woods.

Some animal activists also argue that we should leave the greys alone because, over time, red squirrels will develop immunity to squirrelpox, citing the example of myxomatosis, which was deliberately introduced in the 1950s to reduce Britain’s rabbit population. This disease killed more than 99% of rabbits, but over the past 50 years the population has bounced back. If a virus wipes out its host, it eradicates itself, so, over time, a virus must become less virulent to survive. The host, the rabbit, also develops immunity; survivors pass it to their offspring.

But squirrelpox, as Professor Julian Chantrey explained to me, is different. The virus already has a perfect host – the grey – and so there is no evolutionary imperative for it to become less deadly to reds. “The reds will acquire some immunity, but it’s going to take a long time, with successive epidemics driving the evolution of red resistance,” he said.

I wanted to see for myself whether “cranial despatch” – killing a grey squirrel by hitting it on the head – was cruel. So I drove to North Wales to spend a day with Craig Shuttleworth, an ecologist responsible for the cullers’ greatest achievement: an 18-year effort to clear the island of Anglesey of grey squirrels, which was completed in 2015. Since then, its red population has rebounded from near-extinction to 700.

We met at 7am and headed out in Shuttleworth’s battered blue Land Rover with a mismatched red driver’s door. In the back was a large translucent plastic sack filled with yellow maize and black sunflower seeds, and a 60cm-long, square-edged stick. Shuttleworth is a tall, independent thinker who had resisted the professorships of a conventional academic career. “Scientists are a bit like an under-10s football team,” he said. “They are all around the ball. The clever kid is waiting on the wing for that ball to pop out.” He is now working to clear greys from a small corner of north-west Wales.

He and two other contractors were funded by Red Squirrels United, but Shuttleworth was a little insulted by the fact that the project claims that killing grey squirrels is “a last resort” on the Wildlife Trusts’ website. To Shuttleworth, it’s the only option: “It’s not a last resort. We have no choice, so it’s kill them, kill them, kill them.”

Shuttleworth takes the lives of around 1,000 greys every year, and prefers cranial despatch to shooting. “Scientists in the southern hemisphere have said, ‘You’re pissing in the wind. You’re trapping, which is hugely labour-intensive – why aren’t you using poison?’ But we’re not dealing with a rat eradication on a remote island, we’re dealing with a place where 60 million people live with their pets.”

Fast-moving but zen, like a postman on his rounds, Shuttleworth checked his traps by jumping over slate walls and weaving through overgrown cemeteries, rubbish-strewn parkland and beautiful, mossy oak woodlands. Each trap was a small wire cage containing maize and sunflower seeds. When a squirrel stepped on to a metal plate inside, the door closed. Each had a plastic sheet roof to prevent the animal catching a chill while held inside.

By midday, we’d checked several dozen traps. Each one was empty. Shuttleworth remained calm. He was phlegmatic about his human opponents as well. “As a scientist, the message I give has to be honest, and therefore it’s complicated. [Animal rights groups’] message is simple. They say: ‘You’re lying, it’s bloodlust, it’s cruel, you smash them on the heads – you’re like the Nazis.’ There is all this nonsense.”

Traps laid for grey squirrels in Cumbria. Photograph: Christopher Thomond/The Guardian

Animal Aid fears that psychopaths will volunteer to kill squirrels, and believes that amateur red squirrel conservationists should be vetted, like people who work with children. Shuttleworth finds such fears overblown. Volunteers work in pairs, he told me. “We know where they are and what they are doing. Can we be sure someone doesn’t take a knitting needle and push it through the squirrel in the sack a hundred times? I can’t be sure, but it’s very unlikely. Why would they bother becoming a volunteer with a local group when you can go to a DIY store, buy a trap and legally trap as many grey squirrels as you want?”

Opponents worry that killing greys is not just inhumane but tinged with xenophobia. The late Philip Pauly, an American historian of biology, believed there was an alignment between legislation to curb invasive species in the US, and the imposition of controls on human immigrants. California introduced a plant quarantine law in 1881; a year later came the Chinese Exclusion Act, which barred Chinese immigrant labourers to the US. Over the last two decades, a rising concern for the future of native species in the developed world has coincided with growing nationalist feeling and hostility towards migrants.

Invasion biologists have pointed out that xenophobia towards migrants within the single species of Homo sapiens is not comparable to taking action against invasive species that threaten entire species with extinction. But these academics are increasingly careful with their language. Instead of bandying about terms such as “alien invaders”, biologists now prefer phrases such as the “McDonaldisation of ecosystems” to describe the damage caused by some non-native species. In effect, writes Peter Coates, a professor of environmental history at Bristol University, they are championing a kind of “tasty” local diversity, rather like those campaigning for regionally specific cheeses or varieties of apple against the bland produce of globalised agribusiness. Shuttleworth used a similar analogy. If we don’t control invasive non-native species, he said, “we will end up with a homogenised world where it’s all seagulls, magpies, crows, bracken, rhododendron and grey squirrels. A bit like high streets – they are all the same.”

Animal rights campaigners’ attempts to paint invasive species in a more positive light co-opts the widely accepted view among ecologists today that there is no “balance” of nature, and ecosystems are never stable. But Shuttleworth argues that it is wrong to infer that we should therefore learn to love destructive invasive species. In fact, he said, they prevent dynamism in human-dominated landscapes. For instance, as climate change threatens the survival of some native flora, British foresters will need to bring in new tree species that are better suited to our future climate. But we can’t bring more drought-resistant European hardwoods into Britain because we don’t know if they can survive the bark-stripping of grey squirrels.

Finally, after crashing through a scruffy copse near Bangor, Shuttleworth and I found a grey squirrel in a trap. It dashed from end to end as we approached. Shuttleworth drew out his stick, kneeled down and took perhaps 10 seconds to shift the squirrel from cage to clear plastic bag. Another three seconds, and he had rolled up the plastic, pinning the animal on its side against the earth. Its shiny black eye briefly examined us through the bag. Whack-whack-whack. Three hits. He pulled the squirrel from the bag. There was a spasm in its long rear legs, but the squirrel was dead. He touched its eye. “No eye reflex,” he said.

Why did he hit it more than once? “We don’t want squirrels coming back to life. We don’t want uncertainty,” he said. He was confident every squirrel he killed was dead after the first blow. His hardest kill was at the beginning of his first eradication project. A young squirrel showed no fear when caught in the trap, turning its back on him to hide the food it was eating. Were Shuttleworth’s dreams haunted by all those dead grey squirrels? “Quite the contrary. They become more and more anonymous, and that’s hard for me to accept,” he said. “One of the things I’m acutely aware of is becoming desensitised. How has that changed my relationship with other animals? It hasn’t. I don’t have any urge to kill anything else.”

We reached an area where Shuttleworth only began trapping three days ago, and suddenly found squirrels in five more traps. The second was a female. After killing her, Shuttleworth gently squeezed milk from a teat. “She’s got young. So the young are now waiting.” Could Shuttleworth locate the squirrel’s drey and put its babies out of their misery? “How do you find the drey? If you hit an animal on the road, should you get out, see if it’s lactating and find the nest? It goes on and on.”

Culling grey squirrels in perpetuity to save the reds is a hard sell, as Shuttleworth and Julie Bailey both admit. “I suppose this is a bit like nuclear waste,” said Shuttleworth. “We’ve got to manage how long ‘for ever’ is.”

On Anglesey, he had demonstrated that culling can work and red squirrels will then recover in a grey-free “stronghold”, but he fears that his current project to remove greys from north-west Wales won’t succeed by the end of its three-year funding. Still, he said, it could be successful in the longer term, and Red Squirrels United’s EU funders are interested in whether this eradication project can be transferred to other problematic non-native mammals. For animal lovers horrified by the potential expansion of culls of non-native species, Shuttleworth mounted a more philosophical justification. On Anglesey, he argued, a comprehensive cull ultimately saved more lives than it took; the 6,000 greys culled will be surpassed by the number of reds born in years to come.

Five weeks after visiting Julie Bailey, I phoned for an update. She’d had a “slight fall” and had been in and out of hospital, but had still managed to shoot five greys. One of her rangers was a bit down because there had been another outbreak of squirrelpox and a dying red had to be shot. Did she ever lose energy for this apparently eternal struggle? “Never, ever, ever,” she said. “We don’t lose any faith in it. We know that if we didn’t do what we’re doing, we wouldn’t have any red squirrels. It’s as simple as that.”

Main image: Julie Bailey, administrator of the Penrith and District Red Squirrel Group, on the hunt for greys in Cumbria. Photograph by Christopher Thomond for the Guardian

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