When the final chapter is eventually written about how a rodent invasion sweeping across the United Kingdom was contained, it may be that the turning point came in a small grove of woods in rural Yorkshire.
There, Sarah Beatham and her team from the U.K. Animal Plant Health Agency have been taking aim at the grey squirrel.
"They're an invasive species, and they do a lot of damage to our natural environment," Beatham, a PhD student in biosciences at Durham University, told CBC News as she pounded a stake into the ground to anchor a unique animal trap that could spell doom for the prolific creature.
A much larger and more aggressive cousin to Britain's native red squirrels, greys were brought over a century ago from Canada and the United States by people who thought they would make a pleasant addition to the local U.K. wildlife.
Instead, over the decades, the greys have rampaged through British forests stripping the bark off trees, particularly oaks, and driving red squirrels out of their habitat to the point they're now on the government's "near threatened" wildlife list.
Grey squirrels carry a squirrel pox virus that they've become immune to but is usually lethal to the red squirrels that catch it. By virtue of their much larger size, they can also out-compete the reds for food in areas where they still co-exist.
WATCH | A curious grey squirrel takes the bait:
Red squirrels are about as close to beloved creatures in the U.K. as a rodent can get.
British children grew up reading Beatrix Potter books featuring red squirrels as characters, and Prince Charles has taken a personal interest in the preservation efforts.
The heir to the British throne is the patron of the Red Squirrel Survival Trust, and his son William has been quoted as saying Charles has given individual squirrels names and even allowed them inside the Royal Family's homes on his country estates.
Right now, the three million grey squirrels in the country outnumber the reds by roughly 25 to 1. The Red Squirrel Survival Trust says the latter may become extinct in a decade if more is not done to halt the proliferation of the grey squirrel.
But Beatham says her team has discovered the grey squirrel's weakness.
It's a hazelnut paste that can be mixed with an immunocontraceptive, rendering it impossible for squirrels that eat it to breed.
She says it only took three of the special feeders left out for four days to deliver the mixture to 70 per cent of a local population of 100 squirrels in the Yorkshire region, which would be more than enough to cause a dramatic reduction in births.
"We found that over four days, an average squirrel will come back to the feeder between 80 and 120 times," said Beatham, indicating that getting squirrels to eat the contraceptive mixture wasn't a problem.
The method of injecting birth control directly into captured wild animals has been effective on a localized scale at reducing populations of other species, such as deer.
But in this case, the contraceptive can be delivered to much larger wildlife populations with minimal human involvement.
The contraceptive vaccine being tested in the woods of Yorkshire prompts the squirrel's immune system to produce antibodies against a hormone called GnRH, which is essential for reproduction in males and females.
Animals remain in a non-reproductive state as long as a sufficient concentration of antibodies is present, according to the U.K. Squirrel Accord, the conservation group sponsoring the five-year project, which is in its third year.
Contraception, not cull
"Without this hormone, females do not cycle and males do not produce sperm," said Giovanna Massei, a senior researcher at the Animal and Plant Health Agency and one of the world's top experts in fertility control.
No side-effects were observed in the first trials of the immunocontraceptive, according to the U.K. Squirrel Accord.
But perhaps the biggest challenge in delivering the contraceptives to the Yorkshire squirrels was ensuring that other wild animals didn't eat the nut mixture, too.
The team did that by designing the trap so the weight of the door was heavy enough so that only mature grey squirrels could push it open and not the much smaller red squirrels or other animals.
"We've had some great results so far," said Kay Haw, the director of the U.K. Squirrel Accord.
"Traditional methods [of population control] aren't really working on their own, so I think a lot of people are hoping this will turn the tide."
Those other methods mostly involve trapping or shooting squirrels.
Aside from the public's dislike of the culling of a large number of animals, there's also the expense. The U.K. Squirrel Accord claims it can cost up to 60 pounds (more than $100 Cdn) for every squirrel killed once the cost of manpower is factored in.
Fertility control, on the other hand, has the potential to be done much more cheaply.
Publicly acceptable population control
"Human-wildlife conflicts are increasing worldwide, and there are very few ways of reducing overabundant animals," said Massei, who has devoted much of her career to solving the problem of unwanted human-animal interactions and has worked on similar projects to control populations of wild boar and feral horses.
"Last year, Germany shot 820,000 wild boar."
The hope is the experiments with squirrels in the U.K. can be transferred to other problem creatures.
"Rats and mice and rodents are currently killed with very inhumane methods. Everyone recognizes it."
The nutty traps from Yorkshire aren't quite ready to be deployed on a mass scale as there's more work to do to fine tune the contraceptive mixture and determine the exact dose needed to render the animals infertile. (According to the U.K. Squirrel Accord's FAQ on the project, initial trials of the contraceptive vaccine on rats found that six doses delivered within a month were sufficient to make 60 of the test subjects infertile.)
Haw says this work will give Britain's red squirrels a fighting chance to return to their storied place in the British landscape.
"I think a lot of people are hoping this will really turn the tide."