TO get to the survey site, we walk through the edge of a golf course to the small wood on top of the hill. Today it’s a sunny spot with views over Loch Lomond below.

The air is filled with birdsong, and there’s a gentle breeze in the branches: a fine place, perhaps, for a picnic.

But this is the front line in a fight for survival, and the good news is that battle is starting to be won by one of Scotland’s best-loved animals, the red squirrel.

At the site, Katherine Rennie from the Scottish Wildlife Trust’s Saving Scotland’s Red Squirrels project tops up the nuts in a square feeder box attached to a tree.

She then removes a sticky patch from the lid and replaces it with a fresh one.

“That means if anything goes in to feed they will leave some hairs,” she says. “Then they can be sent away for analysis and hopefully there will be hairs from red squirrels there.”

If there are red hairs it’s more good news for the Trust, volunteers such as Rennie, and their allies on estates along the Highland boundary line, from Helensburgh to Montrose, who are working to protect the red squirrels of the Highlands from the grey invaders.

They are seeing increasing signs that the red squirrel populations are thriving and expanding.

If there are hairs from the non-native American grey squirrel, it is the turn of one of five trappers working for the project along this front line to move in.

Former policeman Cameron McCallum is one of them. His job is to live-trap the greys, take blood samples to detect the pox – and then dispatch the animals.

He emphasises that the culling is carried out swiftly and humanely. “It’s instant. It’s all over in just a just a few seconds,” he says.

It’s a grim process, but there is now real hope that doing it can sustain Scotland’s native red squirrels for the future.

The greys are often welcome as a cute touch of nature in towns and cities, but they push out the native reds: in England the reds have been almost completely lost to this process. They can also damage tree stocks.

Scotland’s red squirrel population is the vast bulk of the 140,000 or so animals left in the UK, with England and Wales having just a handful of tiny, protected, isolated remnants, and one bigger group just south of the border in Cumbria and Northumbria.

Most of Scotland’s reds – 75% of the UK population – are concentrated in the Highlands, with interconnected populations right across the region, where the greys are largely unheard of.

The Trust fought a long campaign along the Border to keep out English greys, which carry squirrel pox, a disease that is part of the mechanism by which greys replace reds. The greys suffer no symptoms but it crushes red populations; the greys then move in to the empty territory and it’s lost to the reds.

Eventually the Border war could not be sustained, and signs of the pox virus began to turn up in the previously uninfected greys of the Central Belt, with animals testing positive for the pox in Glasgow, near Erskine, near Bannockburn and just south of Edinburgh at Roslin.

The Trust had feared that this could be the beginning of the end for the Highland reds immediately to the north, with an inexorable spread through the millions of grey squirrels in the central belt, running right up to the Highlands.

But through analysis of data from years of trapping and culling in southern Scotland they made a remarkable discovery.

To their surprise they found that though greys could not be held back, keeping their numbers low enough meant the reds could still thrive side by side with them.

While some of the reds would still be still hit by the pox, consistent trapping and culling left too few greys to take over the territory left by the dead animals, and so the reds could bounce back, holding their ground and even expanding.

That discovery has led SWT and its partners to establish the new protection zone along the Highland boundary where grey numbers will be kept low enough to protect the Highland red squirrels. Along with the teams from the project based in Balloch and on Tayside, many landowners along the boundary have been given grants to carry out their own trapping work.

Squirrel expert Dr Mel Tonkin is the manager of Saving Scotland’s Red Squirrels. She says the southern campaign failed as pox-infected animals headed north along wooded valleys and other ribbons of woodland such as rail lines from the east coast to Dumfries and Galloway.

“It had got to the point where we hadn’t managed to contain the disease. We had just been outflanked,” she says.

“We knew it was coming. But we did slow the rate of pox spread in Southern Scotland, and the control we had put in had sustained reds, regardless of the occasional outbreak of pox.

“The message that we came out with was if you keep grey squirrel densities very low, you will still get outbreaks of squirrel pox in reds occasionally but it doesn’t dent the populations in the way it would do if the greys could then move into the vacuum and occupy it.

“We are not as worried now about keeping the Highland line totally secure and away from pox. Computer modelling shows a certain level of control will be enough to prevent the spread of pox beyond the Highland line, and we can do that.”

Out at Loch Lomond, Mary-Anne Collis, the conservation officer in charge of the western half of the effort to keep the greys out of the Highlands, manages the work of volunteers such as Rennie.

“If the grey squirrels moved into the Highlands you would see what’s happening in England,” she says. “The reds would be under immense pressure.

“Part of what we’re doing is looking for the squirrel pox in the grey populations and finding out where it is in the landscape, to give us an early warning in case it comes a bit further north.

“A bit of grey control can also give the reds new areas to move into, which is good news.

“It’s fantastic news that by controlling them we can stop the spread.”

Much of the trapping work has been around Dunkeld and Montrose, which have been identified as key northward dispersal points for greys.

As a result of this and the increasing numbers of pine martens – a Scottish conservation success – which prey on grey squirrels, the number of greys is falling back, making the job easier.

It’s not all good news: the greys in the south of Scotland have been spreading, although the red populations there haven’t been affected yet. Tonkin says it’s a fluid situation that can vary from year to year depending on weather and food supplies.

But smaller grey control projects in the south are helping to protect the key remaining red populations there.

SWT as a charity could not sustain the expense of grey squirrel control right across the southern region, but eight red squirrel hotspots have been chosen and a combination of volunteers and landowners is now leading the fight there to keep grey numbers down.

Tonkin added: “You have to have continuing investment and continual control.

“It has to be on a landscape scale. There’s no point in someone just doing their little patch, because it’s not going to work – you have to join in with your neighbours. The squirrels know no boundaries.

“Constant vigilance is needed.”