Migration

For people seeHuman migration;for data seeData migration.

Wildebeestcrossing river inEast Africa

Migrationis whenanimalsmove on a regular cycle. For example,caribouin theArcticgo south in winter and return insummerwhen it is warmer. Manybirdsmigrate, such asgeeseandstorks.

Migration is the travelling of long distances in search of a newhabitat.Thetriggerfor the migration may be localclimate,local availability of food, or theseasonof the year. To be counted as a true migration, and not just a local dispersal, the movement should be an annual or seasonal event.

Many birds fly to warmer places for the winter, as do someinsectssuch as themigratory locust.Young Atlanticsalmonleave the river of their birth when they have reached a few inches (cm) in size.

Many species in the sea have a daily migration.Planktongo up for the day where there is light, and down at night, where they are less easy to find. The many species which feed on them follow them up and down.

Migration is anevolutionaryforce. This is because it is a major source ofnatural selection.The success of migratory animals to make the journey is usually needed for them to reproduce.

Many parts of the world have a strongly seasonalclimate.In order to survive, many species need to breed in one place and, later, eat in another place. The simplest example is the Africanherbivores,who follow the growth ofgrassinEast Africa.This region has seasonalrainfall,and so it has seasonal growth of grass. Theirpredatorsfollow them.

Bird migration

Examples of long-distance bird migration routes

Bird migration is the regular seasonal movement, usually north and south along a 'flyway', betweenbreedingandwinteringgrounds. Many species ofbirdmigrate. Many birds die when they are migrating, and is mainly to get food. It occurs mostly in thenorthern hemisphere,where birds are funnelled on to specific routes by natural barriers such as theMediterranean Seaor theCaribbean Sea.

Distances

TheArctic ternholds the long-distance migration record for birds. It flies betweenArcticbreeding grounds and theAntarcticeach year.Albatrossescircle the earth, flying over thesouthern oceans.TheManx shearwatermigrates 14,000 km (8,700 mi) between its northern breeding grounds and the southern ocean. Shorter migrations are common, includingaltitudinalmigrations up and down mountains like theAndesandHimalayas.

Since the distance that some birds travel on migration can be enormous,why such huge distances? Migration is useful for the birds, because the seasonal differences are to the birds' advantage. So far as is known, migration instincts are inherited: the birds cannot make a personal choice in the matter. All the same, in some species not all migrate. There is a cost to migration in the number of birds which dieen route.There is an advantage only if the birds which migrate have a better chance of leaving descendants. Studies of migration have been done since the 18th century.There is still much we do not know about it.

Historically, bird migration would have started when the continents were close together. It might have started in theUpper Cretaceousor thePalaeocene.Then. as the continents drifted apart, the bird migrations got longer and longer. That makes sense, and probably is what happened. There is no direct evidence.

How do they do it?

Biomagnetismis part of the answer. There are crystals ofmagnetitewhich occur in many birds (and some other organisms).There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).

Related pages

• Shoaling and schooling
• Human migration

References

• Baker R.R. 1978.The evolutionary ecology of animal migration.New York: Holmes & Meyer.