Invasion
A rapid extension of the breeding distribution by natural means, being a form of colonisation. The most spectacular example in recent times has been the invasion of most of Europe by the Collared Dove Streptopelia decaocto, which until about 1930 was confined to the region from Yugoslavia eastwards. The term is also used for an irruption.From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver