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Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons.〔(National Geographic. Why Animals Migrate )〕 To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, such as birds migrating south for the winter; wildebeest migrating annually for seasonal grazing; or a major habitat change as part of their life, such as young Atlantic salmon leaving the river of their birth when they have reached a few inches in size. ==Overview== Migration can take very different forms in different species, and as such there is no simple accepted definition of migration. One of the most commonly used definitions, proposed by Kennedy is Migratory behavior is persistent and straightened out movement effected by the animal’s own locomotory exertions or by its active embarkation upon a vehicle. It depends on some temporary inhibition of station keeping responses but promotes their eventual disinhibition and recurrence. Migration encompasses four related concepts: persistent straight movement; relocation of an individual on a greater scale (both spatially and temporally) than its normal daily activities; seasonal ‘to-and-fro’ movement of a population between two areas; and movement leading to the redistribution of individuals within a population.〔 Migration can be either obligate, meaning individuals must migrate, or facultative, meaning individuals can "choose" to migrate or not. Within a migratory species or even within a single population, often not all individuals migrate. ''Complete migration'' is when all individuals migrate, ''partial migration'' is when some individuals migrate while others do not, and ''differential migration'' is when the difference between migratory and non-migratory individuals is based on age or sex (for example).〔 The three main types of migration are pilotage, compass orientation, and true navigation. While most migratory movements occur on an annual cycle, some daily movements are also referred to as migration. Many aquatic animals make a Diel vertical migration, travelling a few hundred metres up and down the water column, while some jellyfish make daily horizontal migrations, traveling a few hundred metres across a lake. Irregular (non-cyclical) migrations such as irruptions can occur under pressure of famine, overpopulation of a locality, or some more obscure influence. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Animal migration」の詳細全文を読む スポンサード リンク
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