Drift migration is the phenomenon in which migratingbirds are blown off course by the winds at the time they are in flight. It is more likely to happen to birds heading south in autumn because the large numbers of inexperienced young birds are less able to compensate than the adults heading north in spring.
Drift is assisted by disorientating conditions such as mist or drizzle, and can result in large numbers of birds arriving together in an area in which they are not normally seen. This is called a fall.
An example would be an east wind in September blowing Scandinavian migrants such as Bluethroats, Wrynecks and the continental race of Robin onto the east coast of England and Scotland, leading to temporary concentrations of these species at headlands like Spurn.
In the diagram, the red arrow shows the intended route, the blue arrow the wind direction, and the orange arrow the actual migration route.
Many species of land birdsmigrate very long distances, the most common pattern being for birds to breed in the temperate or arctic northern hemisphere and winter in warmer regions, often in the tropics or the southern hemisphere.
An interesting example where a change of migration route has occurred because of such a geographical barrier is the trend for some Blackcaps in central Europe to migrate west and winter in Britain rather than cross the Alps.
An example is the migration of continental populations of Swainson's Thrush, which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago.
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