IrruptionAn irregular movement involving large numbers of birds which takes them into areas where they are not found every year in such numbers. An irruption differs from true migration because it is essentially irregular and does not have a return movement matching the outward movement. Shortage of food on the continent may cause irruptions into the British Isles involving birds such as Waxwings Bombycilla garrulus and tits (family Paridae) but the best known irruptive species is the Crossbill Loxia curvirostra. Irruptions into an area constitute 'eruptions' from the area of origin of the birds.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver