Red squirrels (Sciurus vulgare) are hard to find — they are almost extinct in England. They survive on Anglesey, North Wales and in parts of the Highlands and islands of Scotland, where isolation has saved them from the scourge of squirrel pox, carried by the successful grey squirrel (Sciurus carolinensis) that raids our bird feeders but it’s not as simple as that. Last week we visited some in…