DISPERSAL AND MIGRATIONS 177 



well-defined limits these birds appear to have 

 no settled home. Lastly, we may allude to the 

 Irruptic Movements of birds. These cannot be 

 classed either with normal migration or nomadic 

 migration. They are entirely abnormal move- 

 ments, .only occurring at irregular intervals, and 

 caused by an exceptional excess of individuals 

 breaking forth from congested districts, spreading 

 far and wide, yet never succeeding in colonising 

 new areas. One of the most remarkable instances 

 of this irruptic movement is furnished by that 

 curious bird known as Pallas's Sand-Grouse. 

 Readers may recall its visits to Western Europe 

 and the British Islands a decade ago — an irruptic 

 wave of birds that spread from Central Asia. 

 Similar irruptic waves of Jays and Goldcrests 

 have also been remarked. 



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