Climatic change has a profound effect on the distribution of animals and plants. for instance, between 1850 and 1950, a period of climatic amelioration, Britain gained several new species from southern and eastern Europe, including Collared Dove, Celti's Warbler, Savi's Warbler, Firecrest and Golden Oriole. The situation has been further complicated by the greenhouse effect , with northern and southern species invading Britain. This study traces and explains the changes in bird distribution in the Northern Hemisphere, with special reference to Britain and Europe, relating them not only to climatic change, but also to other environmental factors.