The Highlands of Scotland are the last bastion of the beloved native red squirrel in Great Britain. Now even this most remote of outposts is under siege. North American grey squirrels are using the improved transport links of tree-lined highway A9 as a corridor to advance steadily northwards. These squirrels can carry a disease that threatens the very existence of their beautiful, russet-colored cousins.
Help is at hand, however, from a very unlikely source. The pine marten, a grey squirrel predator, is being encouraged to breed in strategic areas to act as unofficial border guards. The Scottish government agency, Forestry and Land Scotland (FLS), has begun the process of installing thirty-five artificial pine marten dens along grey squirrel migration routes in a bid to protect the native reds.
The need is urgent. For the first time, grey squirrels have been sighted north of the village of Dunkeld on the edge of the Highlands and are advancing on the eastern front up the coast from Angus into Aberdeenshire.
A charitable organization is working with the government to preserve the red squirrels’ remaining habitat from encroaching grey squirrels.
“Saving Scotland’s Red Squirrel Project is working in key areas, defending the highland line to make sure the grey squirrel doesn’t spread beyond that,” said Dr. Emma Sheehy, conservation officer with Saving Scotland’s Red Squirrels. “There is disease in the grey squirrel population, and we want to make sure they are not able to pass beyond that barrier,” she said.
Grey squirrels already have a decades-old colony in the northern city of Aberdeen, which sits above the highland line. The colony was established accidentally when a group of squirrels escaped from a children’s petting zoo in the 1970s. The Aberdeen colony currently has a population of a few thousand, disease-free squirrels. “But, the risk is that, if unmanaged, the grey squirrels in Aberdeen will continue spreading, and if they connect with the [grey squirrel] population in the south that do have the disease, then the infectious disease would go into the Grampian and Highlands red squirrel population,” Dr. Sheehy said. That would be “really devastating.”
“Removing the grey squirrels in the northeast is a really key piece of red squirrel conservation in the UK,” she added.
The disease in question—squirrel pox—is what makes the “squirrel war” urgent.
Seven out of ten of Great Britain’s grey squirrels carry squirrel pox, a highly contagious, viral disease so deadly it can kill a red within five days, but to which the greys are immune. Gareth Ventress, an environment forester for FLS, who has been installing pine marten boxes, is confident that this culling-by-predator is necessary to protect an indigenous species from extinction.
“If grey squirrels were a native species and were outcompeting another native species, that’s just nature and we would just be looking at habitat management rather than culling,” he said. “But if you have an invasive species outcompeting the native species, and then you add the squirrel pox, it’s really about stopping them getting into the Highlands and trying to hold the highland line.”
He adds that no matter how successful the pine marten patrols are, they can’t eradicate grey squirrels from Scotland; the country’s southern cities have extremely large populations. In contrast, in England, red squirrels are almost extinct, with only a few pockets surviving in parts of Wales.
With a team of three or four co-workers, Mr. Ventress selects woodlands that already contain pine martens and are “grey squirrel-friendly”—an attractive habitat with abundant food sources. Mr. Ventress finds trees that are about 1.2 miles (2km) apart and ropes his boxes to the trees, so they are high off the ground.
Many trees on grey squirrel migration routes are not yet old enough to have the cavities pine martens need for their nests, so the man-made boxes are intended to replicate what a pine marten would look for in a mature woodland.
“We put the boxes up in the air because the pine marten needs to feel safe and warm. If we didn’t have these boxes, the pine marten would end up in a buzzard nest, which is open to the elements and not that great if you have young kits for sleeping,” Dr. Sheehy said. Nests on the ground are vulnerable to attack by badgers and foxes, she said, “so what you want are arboreal cavities, which these boxes are replicating.”
When pine martens are established in greater numbers, they will reduce the grey squirrel population by killing them or scaring them off, Mr. Ventress said. This is because if there are high numbers of predators around, it creates a “landscape of fear” that will impact the surviving squirrels’ “fitness and breeding success,” he explained.
Dr. Sheehy stressed that this is a long-term project—any impact on grey squirrel numbers is not expected to be seen for at least a decade.
“Pine martens, as a species, don’t move quickly. Things take a really long time, and they are really slow breeders,” she said. “For example, it would be 2025 at the earliest before a female [pine marten] born this spring would have her own offspring,” Dr. Sheehy said. “It would be even longer before the next generation comes along. Things don’t happen very quickly.”
“Pine martens, as a species, don’t move quickly. Things take a really long time, and they are really slow breeders.”
However, evidence from Ireland, which like the UK is one of the few countries in the world that has both types of squirrels, indicates that by encouraging pine martens, the grey squirrel population can be reduced to a great extent. “It’s taken about ten to fifteen years in Ireland, and we don’t know how far behind Scotland is,” Dr. Sheehy said.
To the lay person, one of the most puzzling elements of this story is why pine martens are a threat to one type of squirrel but not both.
Dr. Sheehy believes that while greys are “naïve” to the threat of pine martens as a predator, reds evolved together with the pine martens over millennia and are aware of the danger they pose. “One experiment carried out showed red squirrels became much more vigilant when pine marten skat was sprayed on their feeders, but grey squirrels acted like nothing had happened. This indicates they aren’t picking up the cue a dangerous predator is around, which makes them far more susceptible,” Dr. Sheehy said.
Another advantage red squirrels have are their much smaller heads, allowing them to squeeze into tighter spaces and out of the reach of a ravenous pine marten.
There are several charities in each nation across the United Kingdom set up to stop the red squirrel from going extinct.
The townsfolk of Pitlochry and Dunkeld are generally in favor of the introduction of the pine marten dens, but not everyone is on board.
There are some people who disagree with the plan, “as we are killing animals and people don’t like that,” said Mr. Ventress. Also, “some people were a bit anti having pine martens because of their effect on the songbirds in spring and the potential effect on red squirrels as well. It’s not everybody’s cup of tea, that’s for sure.”
But others support this novel effort to save the red squirrels, and many have already volunteered to take part in the trapping and culling of the greys, Mr. Ventress said.
*Gordon Cairns is a freelance journalist and teacher of English and Forest Schools based in Scotland.