Introduced species often become pests, causing damage to agriculture or forestry, and/or the extinction of native species. In Britain and parts of North-Italy, the introduced American grey squirrel replaced the native red squirrel. The process of this replacement is still unknown. By comparing the behaviour and reproduction between two control areas (only red squirrels) and two experimental areas (red and grey squirrels), two hypotheses will be tested. I expect grey squirrels to compete with red squirrels for food resources and to kill their young in the nest. I therefore predict: that red squirrels will spent less time foraging, eat fewer energy-rich tree seeds, and spent more time interacting in the Experimental areas (hypothesis l); and that in the presence of greys, a smaller proportion of female red squirrels will wean offspring (hypothesis 2). The incidence of diseases will also be examined and compared between control and experimental areas.