Grey squirrel (Sciurus carolinensis) eating a nut. This rodent is native to the midwestern USA and eastern Canada. It was introduced to the UK in the early 20th century where it has largely displaced the indigenous red squirrel population (Sciurus vulgaris). Grey squirrels are large rodents, growing up to 50 centimetres in length. Their large size and consequent acquisition of a large share of food is thought to be one of the reasons for their dominance over the native red squirrel. Photographed in Regent's Park, London, UK.