Reds are smaller and lighter, and can reach branches where Pine Martens can't get them, and also spend less time on the ground where they are vulnerable. Whereas the Greys are bigger, and spend more time vulnerable to the Pine Martens.
The balance of power is fascinating and explained well in the article where benefits, and even negatives of Pine Martens on Reds can depend on tree type and variety, and also the supply of other sources of food such as voles.
Pine martens to be used as ‘bouncers’ to keep grey squirrels out of Highlands
Pine martens are to be deployed as wildlife bouncers along the east coast of Scotland and the A9 corridor to halt the northward march of grey squirrels.
More than 35 artificial pine marten dens are being installed by Forestry and Land Scotland (FLS) at strategic locations on the grey squirrels’ path of northward migration in an attempt to save the Highland red squirrel populations.
Red squirrels have been driven out of most of the British countryside since the introduction of greys in Victorian times, mainly because greys carry the squirrelpox disease, which leaves them unharmed but is fatal to the reds.
But recent research has found that the return of the predatory pine marten has reduced grey squirrel populations. Because the native red squirrels and martens evolved alongside each other over thousands of years, the reds appear less affected by martens.