When greys arrive into an area of woodland the resident red squirrels usually disappear within 5-15 years. No-one is sure exactly how this happens but it is believed that the grey can out compete the reds in accessing food and as a result the reds starve and become unable to gain weight to successfully breed.The grey squirrels also act as a carrier of the pox virus. See Squirrel Pox Virus
Unfortunately supplementary feeding on its own is not enough to save the red squirrel. A targeted grey squirrel control programme is essential to ensure the red squirrel population survives. In Tollymore Park Wildlife Officers humanely cull the grey squirrels in accordance with Industry Best Practice and Forestry Commission Guidelines, initially this was to protect the timber trees, but is now also to conserve the red squirrels there. Grey squirrels are known to significantly damage trees by stripping bark off the trunk. This can have significant impact on the profitability of commercial broadleaf timber production.
A healthy red squirrel
'I'm Bigger than you red'
A red squirrel with pox lesions