Reverse migration
Encyclopedia
Reverse migration is a phenomenon in bird migration
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

. Although some large birds such as swan
Swan
Swans, genus Cygnus, are birds of the family Anatidae, which also includes geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae...

s learn migration routes from their parents, in most small species, such as passerine
Passerine
A passerine is a bird of the order Passeriformes, which includes more than half of all bird species. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders: with over 5,000 identified species, it has roughly...

s, the route is gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

tically programmed, and young birds can innately navigate to their wintering area.
Sometimes this programming goes wrong, and the young bird, in its first autumn, migrates on a route 180o from the correct route. This is shown in the diagram, where the breeding range of a hypothetical Asian species is shown in yellow. The correct migration route to the wintering grounds in south east Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

 is indicated in red.

If a bird sets off in the opposite direction, shown by the orange arrow, it will end up in Western Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 instead of South East Asia. This is a mechanism that leads to birds such as Pallas's Warbler
Pallas's Warbler
The Pallas's Warbler or Pallas's Leaf Warbler is a leaf warbler which breeds in southern Siberia , northern Mongolia, and northeastern China...

 turning up thousands of kilometres from where they should be. Keith Vinicombe
Keith Vinicombe
Keith E. Vinicombe is a British ornithologist and writer on bird identification.Vinicombe is best-known for his first book, the Macmillan Field Guide to Bird Identification...

 suggested that birds from east of Lake Baikal
Lake Baikal
Lake Baikal is the world's oldest at 30 million years old and deepest lake with an average depth of 744.4 metres.Located in the south of the Russian region of Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast, it is the most voluminous freshwater lake in the...

 in Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 (circled) could not occur in western Europe because the migration routes were too north-south.

Most of these lost young birds perish in unsuitable wintering grounds, but there is some evidence that a few survive, and either re-orient in successive winters, or even return to the same area.

An article in British Birds
British Birds (magazine)
British Birds is a monthly ornithology magazine that was established in 1907. It is now published by BB 2000 Ltd, which is wholly owned by The British Birds Charitable Trust , established for the benefit of British ornithology...

by James Gilroy and Alexander Lees in September 2003 suggested that misorientation occurs in random directions, but differential survival in different directions combined with asymmetric observer coverage leads to the observed distribution of vagrants. Although "reverse migration" per se certainly does occur, and has been documented well in numerous instances, it does not account for the occurrence in Europe in autumn of Asian vagrants that winter in East Africa, for instance, or the rarity of many southern European species in the UK that winter in West Africa.
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