Some populations of the North American caribou, for example, many herds in the subspecies, the barren-ground caribou, and some woodland caribou in Ungava and Labrador, migrate the farthest of any terrestrial mammal, travelling up to 5,000 km (3,100 mi) a year, and covering 1,000,000 km2 (390,000 sq mi). Other North American populations, the woodland caribou (boreal) for example, are largely sedentary. Smaller herds and island herds like R. t. pearsoni make the shortest migrations least.
Normally travelling about 19–55 km (12–34 mi) a day while migrating, the caribou can run at speeds of 60–80 km/h (37–50 mph). 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. During autumn migrations groups become smaller and they begin to mate. During the winter, migratory herds travel to winter feeding grounds along coastlines in the tundra above the tree line. Below the tree line they shift to the forest for winter feeding. By spring, groups leave their winter grounds to go to the calving grounds. A caribou can swim easily and quickly, normally at 6.5 km/h (4.0 mph) but if necessary at 10 km/h (6.2 mph), and migrating herds will not hesitate to swim across a large lake or broad river.