Migration is defined as the regular, usually seasonal, movement of all or part of a population of animals. Many different animals migrate, including birds, hoofed animals, bats, whales, seals, and salmon. One-way movement of animals that do not return is called emigration. Emigration is due to different causes and is not considered migration. Also, the regular daily movements that many animals undertake are not considered migration.

Most birds and mammals that migrate follow an annual or seasonal pattern related to cyclic variations in temperature, vegetation, or precipitation. Salmon and some other fishes do not migrate annually. Instead, they return to their place of birth in order to reproduce. Some ethologists do not consider these movements migration but reserve the term for cyclic movements. In some parts of the world, animals will suddenly move into a new area temporarily. These sudden and temporary movements are called irruptions. Irregular movements...