The camera trap on a beech at compartment 47 in Kielder Forest captured the image at 3:14 on the morning of 12 March 2019. A pine marten, female, in good condition, looking directly at the lens. It was the first confirmed photograph of a wild pine marten in Northumberland in over a century.

Dr. Kate Stringer, a mammalogist with Newcastle University, was the first person to see the image, on her laptop in a hotel room in Hexham, twelve hours later. She remembers, she says, putting her hand over her mouth.

The pine marten, Martes martes, is a member of the weasel family, about the size of a small domestic cat, with chocolate-brown fur and a creamy yellow throat patch unique to each individual. It was once widespread across British woodland.

By 1900, after centuries of persecution for its fur and as a perceived threat to game, it had been driven to extinction across nearly all of England. A relict population held on in the Scottish Highlands. Smaller relicts persisted in remote parts of Wales and Cumbria, though many doubted they survived at all.

Stringer's study, the Northern England Pine Marten Project, began as a doctoral thesis in 2019 and has continued, with rolling funding from Forestry England, the Vincent Wildlife Trust, and a private donor, through 2026.

The 2019 photograph was the first of what became a slow trickle of detections. By 2021 the team had confirmed five distinct individuals through camera traps and hair tube DNA. By 2023 the count had reached fourteen. The latest figure, published last month in Mammal Communications, stands at thirty-one confirmed individuals across the Kielder, Wark, and Spadeadam forest blocks, spanning the Anglo-Scottish border.

The animals are almost certainly descendants of a population released in the Forest of Dean in 2019 and 2021 by the Vincent Wildlife Trust, combined with natural recolonisation from Galloway in southwest Scotland.

Stringer is careful with the word success. Pine martens, she says, do not declare themselves succeeded. They simply, eventually, are present.

The study has involved a small field team and a great deal of walking. Stringer's principal field assistant, Jonny Holroyd, a former gamekeeper from the North Pennines, has walked an estimated six thousand kilometres through Kielder forest tracks since 2020. He carries a stack of camera trap batteries, a roll of cable ties, a thermos, and a sandwich.

“The study has involved a small field team and a great deal of walking.”

He has seen a live pine marten, in the wild and not on a camera, three times.

The first sighting, he says, was a head and shoulders above a moss-covered log at a place called Bakethin in November 2021. The animal looked at him for perhaps four seconds and then was simply gone. He sat down on the path and did not move for ten minutes.

The recolonisation has had effects beyond the pine martens themselves. A 2017 study in Ireland, led by Dr. Emma Sheehy and published in Biological Invasions, had previously demonstrated that the presence of pine martens correlates strongly with declines in introduced grey squirrels, Sciurus carolinensis, and recovery of native red squirrels, Sciurus vulgaris.

The mechanism is not fully understood. Pine martens do prey on grey squirrels directly. They may also alter grey squirrel behaviour in ways that reduce their breeding success. Red squirrels, lighter and more cautious, appear less vulnerable.

Stringer's team has begun documenting an apparent shift in the squirrel populations of Kielder. A 2025 survey, using citizen science data alongside transect counts, recorded grey squirrel sightings down 38 percent from a 2018 baseline in areas with confirmed pine marten activity. Red squirrel sightings were up 22 percent over the same period.

She cautions against attributing causation too neatly. There are confounding variables. Squirrelpox outbreaks. Habitat changes. The unusual winter of 2022.

But the trend is in the expected direction, and other recolonising populations in Wales and Yorkshire are showing similar patterns.

The wider ecology of the forest is also shifting in subtle ways. Wood mice and bank vole populations, monitored quarterly through small mammal trapping, have declined in areas of high marten activity. Tawny owls, which compete with pine martens for some of the same prey, show no clear response.

Kielder itself is an unlikely place for a predator comeback. The forest is the largest planted woodland in England, established in the 1920s and 1930s on land cleared of hill farms during the agricultural depression. It is monocultural in places, dominated by Sitka spruce, and was for decades regarded by ecologists as a green desert.

The pine martens have demonstrated, with some humour, that this view was incomplete. They tolerate spruce plantations. They den in old stone shielings and in cavities in mature beech and oak along the burn margins. They feed on small mammals, fledgling birds, eggs, bilberries, and, in autumn, rowan berries in conspicuous quantity.

Their scats, which Stringer's team collects and analyses for DNA and diet, are often vividly orange in October.

Holroyd has a theory, unpublished and unprovable, that the pine martens of Kielder are slightly larger and slightly bolder than their Scottish ancestors. He bases this on the size of the scats and on something in the eyes of the animals he has seen.

Stringer treats this with appropriate scepticism and an affection she does not bother to hide.

The study will continue until at least 2028. The next phase will involve fitting GPS collars to a small number of adults to map home ranges and dispersal.

On a cold evening in March, Holroyd checks a camera trap above a small stone bridge over the Bakethin Burn. He swaps the SD card, replaces the batteries, and walks back down the track in the dark. Somewhere, a tawny owl calls.

He does not see a pine marten tonight. He does not need to. He knows, now, that they are here.