when only a day old. [ 43 ] During the spring migration smaller herds will group together to form larger herds of 50,000 to 500,000 animals but during autumn migrations, the groups become smaller, and the reindeer begin to mate. During the winter, reindeer travel to forested areas to forage under the snow. By spring, groups leave their winter grounds to go to the calving grounds. A reindeer can swim easily and quickly, normally at 6.5 km/h (4.0 mph) but if necessary at10 km/h (6.2 mph), and migrating herds will not hesitate to swim across a large lake or broad river. [ 2 ] Originally, the reindeer was found in Scandinavia , eastern Europe, Greenland , Russia, Mongolia , and northern China north of the 50th latitude . In North America, it was found in Canada, Alaska (USA), and the northern conterminous USA from Washington to Maine . In the 19th century, it was apparently still present in southern Idaho . [ 2 ] Even in historical times, it probably occurred naturally in Ireland . During the late Pleistocene era, reindeer were found as far south as Nevada and Tennessee in North America, and as far south as Spain in Europe. [ 40 ] [ 45 ] Today, wild reindeer have disappeared from many areas within this large historical range, especially from the southern parts, where it vanished almost everywhere. Large populations of wild reindeer are still found in Norway , Finland , Siberia , Greenland, Alaska, and Canada. There