Some populations of the North American caribou [[Animal migration|migrate]] the furthest of any terrestrial mammal, traveling up to {{convert|5000| km|abbr=on}} a year, and covering {{convert|1000000| km2|abbr=on}}.<ref name=IUCN/><ref>[http://www.mrnf.gouv.qc.ca/english/wildlife/maps-caribou/ Caribou Migration Monitoring by Satellite Telemetry]</ref> Other populations (e.g., in Europe) have a shorter migration, and some, for example the subspecies ''R. t. pearsoni'' and ''R. t. platyrhynchus'' (both restricted to islands), are residents that only make local movements.
Normally travelling about {{convert|19|-|55| km|abbr=on}} a day while migrating, the caribou can run at speeds of {{convert|60|-|80| km/h|abbr=on}}.<ref name=IUCN/> 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 {{convert|6.5| km/h|abbr=on}} but if necessary at {{convert|10| km/h|abbr=on}}, and migrating herds will not hesitate to swim across a large lake or broad river.<ref name=IUCN/>
