Migratory behavior

Migration occurs when living organisms move from one biome to another. In most cases organisms migrate to avoid local shortages of food, usually caused by winter or overpopulation. Animals may also migrate to a certain location to breed, as is the case with some fish.

Animal migration
The species that periodically migrate are called migratory, those that do not are called resident (or sedentary).

Whales and other animals, such as gnus, butterflies, moths, salmon, eels, and lemmings are known to migrate. The periodic migration of plagues of locusts is a phenomenon recorded since Biblical times. Occasionally (and usually unexplainedly) an animal may deviate markedly from its customary migration. A widely publicized incident involved Humphrey the whale, a humpback whale who errantly entered San Francisco Bay.

Bird migration
Bird migration is common. The longest known migration of a bird is that of the Arctic Tern, which migrates from the Arctic to the Antarctic and back each year. Flyways are routes that certain bird species take to migrate.

Human migration
Human migrations also happen on a large scale, in history and in modern times. Seasonal human migration is very common in agricultural cycles.

In archaeology, migrationism describes an interpretative framework where all major cultural changes are explained by large-scale movements of people.

Epidemiology and genetics
In population genetics and evolutionary biology, gene flow describes to migration's effect on the genetic makeup of the population migrated into.

Modern transport, particularly the volume and speed of air transport has facilitated the rapid migration of bacteria and viruses which cause diseases. One of the earliest examples is the infamous plague epidemics or "Black Death" which arrived in Europe along trade routes via the Middle East from the Orient. More recently, virulent strains of influenza and AIDS.

"Migration" in other contexts
The term "migration" may also be applied to the movement of non-living things:


 * In geophysics, migration is a process which keeps in account the correct positions of samples in sections with dipping reflectors and structural complexity.


 * In control systems when a set of instructions or programs, i.e. PLC (programmable logic controller) programs, are moved from an old platform to a new. In this way the needed reengineering can be kept to a minimum.


 * In computer clusters, process migration refers to what happens when one instance of a program is being moved from one of the cluster computers to another.