Seasonal migrations occur in many species of insects, birds, marine mammals, and large herbivorous mammals. These migrations often provide the animals with more favorable conditions of temperature, food, or water. Many birds and a few bats of cold and temperate regions migrate to warmer areas during the winter. Herbivores of cold regions, such as wapiti (elk), caribou, and moose, have summer and winter ranges; many herbivores of warm regions, such as the African antelopes, migrate seasonally to avoid drought. These migrations may involve a change of latitude, of altitude, or both.
In many cases the chief function of seasonal migration is to provide a suitable place for reproduction, which may not be the place most suitable for the feeding and other daily activities of adults. Hundreds of thousands of gnus (wildebeests) of E Africa take part in annual migrations to calving grounds. Many fishes migrate to spawning grounds, and in some cases this involves a change from saltwater to freshwater (e.g., salmon) or vice versa (e.g., freshwater eels). Sea turtles, seals, and many sea birds come ashore to breed, and most amphibians gather near water at the breeding season. Fur seals and many whales make ocean voyages of thousands of miles to their breeding grounds, the former coming ashore on islands. Such migration is seriously affected by the increasing rate of destruction of natural habitats.
The term emigration refers to irregular movements out of an area, with no return. When such emigration is the result of sudden, explosive population increase, it is called an irruption. Irruptions are common among small rodents, notably lemmings, and various species of birds and insects. The mass movements of the so-called migratory locusts of N Africa ( Locusta ) and North America ( Melanoplus ) are actually irruptions; however, the N African desert locust ( Schistocerca ) makes true migrations between its winter and summer breeding grounds.
Another type of one-way travel is the regular dispersal of the young of most species. The simplest type of regular migration is the diurnal movement of some marine microorganisms from one depth to another in response to light changes. Certain marine invertebrates, such as the palolo worm (see Annelida), have a monthly migration pattern influenced by the phases of the moon.