Weather movementA shift in the winter quarters of a population of birds in response to adverse weather, usually snow or hard frost. When conditions improve the birds may or may not return to their original wintering area. The movement can involve thousands of birds travelling hundreds of kilometres, but it cannot be regarded as true migration because of its irregularity and the lack of a definite return movement. The Lapwing Vanellus vanellus often undertakes conspicuous weather movements, large number crossing the North Sea when icy conditions affect the European continent, and moving on along with many British birds to Ireland, western France and even Spain if Britain itself has a very cold spell.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver