Grey squirrels are notorious for displacing red squirrels in European woodlands. They out-compete the native reds for food, feeding more at ground level and being able to digest acorns, which the reds can't. They also carry a deadly pox virus which does not affect them. Grey squirrels were introduced to the UK from the USA in the late 19th or early 20th century. They're now widespread throughout England south of Cumbria and Wales, and common in local pockets in Scotland. They are absent from the rest of mainland Europe, except for small localised populations in Italy.

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