Normally travelling about a day while migrating, the caribou can run at speeds of . Young caribou can already outrun an Olympic sprinter when only a day old. 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 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 about but, if necessary, at and migrating herds will not hesitate to swim across a large lake or broad river.