Grey squirrels are being culled in some areas of Scotland
Concerns have been raised over the further spread of grey squirrels after sightings in the Cairngorms.
The Scottish Wildlife Trust (SWT) said greys were now spreading so far north its programme of culling them was the only way to save the native reds.
Evidence suggested the animals were travelling along the Tay, Dee and Don rivers towards the Highlands.
SWT has said it wants to cull all greys north of a line between Argyll and Angus to protect red squirrels.
It is estimated there are only 121,000 red squirrels left in Scotland.
Grey squirrels were introduced into the UK by the Victorians and have already wiped out most of the reds in the central belt.
Wildlife experts have predicted reds will become extinct within 50 years if there is no intervention.
Traps have been laid to capture greys where they have been sighted.
They are then killed by a blow to the head.
Mel Tonkin, project manager with the Scottish Wildlife Trust, said: "We have got mountain ranges which help slow the spread of grey squirrels.
"If we target particular dispersal routes that take them around or through those mountains, we should be able, by culling just a small number of greys, to stop them getting ever further north."
On the River Tay, sightings of greys have been reported as far north as Pitlochry.
In Aberdeenshire, they have been spotted at Alford on the Don and Braemar on the Dee.
Braemar is in the Carirngorms National Park and there has been concern at how close the squirrels are to the Highlands.
Other communities along the Dee have also reported an increase in sightings.
Cecilia Rogers, from Banchory, said: "I personally am very concerned. I know people say they have a right to be here but I feel the reds have a right to be here as well.
"We can't have them both, that has been proved throughout the country. You've got to make a choice, one of the other.
"My choice would be for the reds, they were here first."