Vagrants are birds that appear from time to time far removed from their usual haunts, in localities where they do not normally breed, winter or occur on passage. They are seen most numerously on islands near the edges of continents, or in other coastal localities. Their appearance is to some extent predictable, and particular species may turn up at more or less the same places and in the same months in different years. Almost all vagrants derive from long-distance migratory populations, and occur away from their regular range at their normal migration seasons. Their appearance can often be linked to natural processes, such as post-fledging dispersal (in any direction) and off-course drift by wind.

For some species, vagrancy can be linked to apparently recurring 'flaws' in the inherent timing and directional control mechanisms. One apparent error in timing produces longer-than-usual migrations, in which birds fly in the usual direction but much further than usual. This leads to long-distance overshooting, and the appearance of individuals far beyond their regular breeding or wintering areas. Spring overshoots are often associated with prolonged periods of favourable tailwinds.

In many species, juveniles migrating for the first time show a greater spread of directions than adults, so that juveniles more frequently end up outside the usual route for their population. In some species, however, distinct directional errors seem to occur, including: (a) mirror-image migration in which birds take the same angle as normal, but on the wrong side of the north-south axis from either the breeding or the wintering area; and (b) reversed-direction migration, in which birds take the spring direction in autumn, or the autumn direction in spring. Again, both types of movement can be assisted by winds, with birds travelling for many hundreds of kilometres in a wrong (but consistent) direction. Possible bias in observer coverage throws doubt on some apparent examples of mirror-image and reversed-direction migration, and neither mechanism can be considered as proven or disproven. There can be no doubt, however, that vagrancy is a natural phenomenon resulting from several different types of causal factors.

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Part Two

The Timing and Control of Migration

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Ringed Plover Charadrius hiaticula incubating

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