Migrazione

In zoologia, **lo spostamento periodico**, spesso **stagionale**, di una popolazione o di parte di essa **verso climi o ambienti diversi**, che comporta il successivo ritorno (degli stessi individui o delle generazioni successive) al luogo di partenza, ed è in genere **determinato** **dalla comparsa di stimoli specifici**, dovuti a variazioni delle condizioni **climatiche** o **ambientali** e alle conseguenti modificazioni **fisiologiche**; si differenzia dall’ // emigrazione //  (che non implica necessariamente il ritorno al luogo di partenza), dall’ // invasione //  (che consiste nella comparsa irregolare, per tempi e luoghi, di un alto numero di individui), dall’ // espansione // dell’areale di una specie e dalla // dispersione //  degli individui giovani verso nuovi territorî; si verifica in tutte le classi animali e risponde all’esigenza di raggiungere siti adatti alla **riproduzione** (per es., nelle tartarughe marine, nei pesci anadromi e catadromi, negli uccelli), di seguire fonti alimentari (per es., negli ungulati, nei cetacei, nelle locuste), di spostarsi nei luoghi di ibernazione o svernamento (per es., nella farfalla monarca). Le migrazioni possono seguire gradienti altitudinali o di profondità (come nel caso delle // m // . __// verticali // __ __del plancton__, che seguono un ciclo giorno-notte), oppure gradienti latitudinali; in quest’ultimo caso, caratteristico della maggior parte delle specie, sono seguite **// rotte di migrazione // ** fisse, le quali implicano una capacità di orientamento mediante sistemi di **riferimento esterni** (topografici, climatici, chimici, astronomici, ecc.) o interni (sensibilità al campo magnetico terrestre).

Migration occurs when living things move from one biome to another. In most cases, organisms migrate to avoid local shortages of food, usually caused by winter. Animals may also migrate to a certain location to breed. Bird migration is commonis 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. Whales, butterflies]] , moths , and lemmings are also known to migrate. The periodic migration of plagues of locusts is a phenomenon recorded since Biblical times. 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. Migration is the movement of animals in large numbers from place to place. Many different types of animals migrate, and depending on the animal, the trip may last as long as an entire lifetime or as short as a few months. Some animals follow a specific route during the course of a migration, while others may follow a path less direct. Many stops may be made along the way depending on the breeding or feeding preferences of the migratory animals.

Migration is the periodic movement of an animal from the place where it has been living to a new area and its subsequent return journey to the original home. When animals migrate, it is usually to find abundant food and a good place to breed. These seasonal movements of animals are one of the most incredible elements of nature. Migrating animals usually use the same routes year after year--from generation to generation. Land animals cross mountains, rivers, and vast tracks of land. Birds, bats, and insects fly long distances, sometimes crossing entire continents or oceans. Swimming animals frequently migrate half way across the world. The movement of migratory animals usually corresponds with seasonal changes. Many animals migrate to northern regions during summer months. The long summer days in the northernmost portions of the world ensure a good food supply. As fall and colder weather approaches, many animals migrate south to find warm winter weather and available food. Some animals migrate annually with an outward and a homeward journey made in a single year. Some animals' patterns of migration are linked to weather patterns---their movements are dependent on rainfall and the availability of green vegetation. A few animals take several years to complete their migratory cycles. Many scientists view animal migration as an adaptation. Animals that have learned to move to optimal environments are the ones who have survived to continue their species.