Migration is whenanimals move on a regular cycle. For example,caribou in theArctic go south in winter and return insummer when it is warmer.
Migration is the travelling of long distances in search of a newhabitat. Thetrigger for the migration may be localclimate, local availability of food, or theseason of the year. To be counted as a true migration, and not just a local dispersal, the movement should be an annual or seasonal event.
Manybirds migrate to warmer places for the winter, as do someinsects such as themigratory locust. Young Atlanticsalmon leave the river of their birth when they have reached a few inches (cm) in size.[1]
Many species in the sea have a daily migration.Plankton go 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 anevolutionary force. 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 ofgrass inEast Africa. This region has seasonalrainfall, and so it has seasonal growth of grass. Theirpredators follow them.[1]
Bird migration is the regular seasonal movement, usually north and south along a 'flyway', betweenbreeding andwintering grounds. Many species ofbird migrate, and many birds die when they are migrating. The main reason to migrate is to get food. Birds mostly migrate in thenorthern hemisphere, where birds are funneled on to specific routes by natural barriers such as theMediterranean Sea or theCaribbean Sea.
TheArctic tern holds the long-distance migration record for birds. It flies betweenArctic breeding grounds and theAntarctic each year.Albatrosses circle the earth, flying over thesouthern oceans. TheManx shearwater migrates 14,000 km (8,700 mi) between its northern breeding grounds and the southern ocean. Shorter migrations are common, includingaltitudinal migrations up and down mountains like theAndes andHimalayas.
Since the distance that some birds travel on migration can be enormous,[2] 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.[3] 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 Cretaceous or 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.
Biomagnetism is part of the answer. There are crystals ofmagnetite which occur in many birds (and some other organisms).[4] There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).[5]
Taken together these data suggest a previously unknown two-molecule sensing mechanism in which KCNJ15/Kir4.2 couples with polyamines in sensing weak electric fields.
