Migration is a product of natural selection, leading species in seasonal environments to adopt movement patterns that enable individuals to survive and breed most effectively. Autumn migration gives improved winter survival through providing access to greater food availability in winter quarters. Spring migration gives improved breeding success, through greater seasonal food availability in summer quarters. The roles (if any) of predation, parasitism and competition in the evolution of migration are less obvious. Migratory birds seem to possess no major adaptations that resident birds lack; they differ only in the degree of development and modification of particular features, whether concerned with orientation, physiology or morphology. They include an internal clock and other timing mechanisms, the ability to deposit extra fat for the journeys, and to orientate and navigate over long distances, west-east as well as north-south.

Some bird populations are obligate migrants and others are obligate residents. Yet others are partial migrants in which some individuals stay year-round in their breeding areas while others migrate elsewhere. Partial migration can apparently arise in two ways, either with a mixture of obligate migrants and obligate residents in the same population, or with the entire (or part of the) population consisting of facultative migrants in which migration is optional, with individuals varying their behaviour according to conditions at the time. However, because migratory behaviour shows continuous, rather than dichotomous variation, both obligate and facultative phenotypes may also exist in the same population.

Breeding experiments on captive birds have shown that various aspects of migratory behaviour, such as the amount, timing and duration of migratory activity, along with directional preferences, are under genetic control, and can be altered by selection to give substantial changes within a few generations. Such changes may be facilitated by genetic correlations between some traits, so that selection for one trait can alter others at the same time. In particular, the occurrence of migratory behaviour is probably controlled by the same genes that control migration distance, operating via the amount of migratory activity. Directions are evidently controlled independently, and individual variation in directions is less in long-distance than short-distance migrants.

Since 1960, a natural change has occurred in the migratory behaviour of Blackcaps Sylvia atricapilla: some central European birds have changed their direction of migration, and now travel west-northwest to winter in southern Britain (the local British breeding birds continuing to migrate southwest to Iberia). Breeding experiments have confirmed that this new migratory behaviour is inherited. Further evidence for the genetic control of migration has come from studies of the year-to-year consistency (repeatability) in the migratory behaviour of individuals, and from the resemblance in behaviour between genetically related individuals (heritability), both in captive and in wild birds. Most migratory traits that have been examined show moderate to high heritability.

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