Around the turn of the century the North American grey squirrel, Sciurus carolinensis, was releasted from various sites in Britain. Since then the grey squirrel has successfully spread to colonize much of England, Wales and the Scottish Lowlands. Simultaneously, the indigenous red squirrel, Sciurus vulgaris, has disappeared from these localities. Three hypotheses have been put forwarded to account for this phenomenon: (i) competition between the reds and the greys, (ii) environmental chang...