96 THE MIGRATIONS OF BIRDS 



the most potent cause that may be given, and has 

 certainly been the active factor in many cases. It 

 may be mentioned particularly in connection with 

 the periodic invasions of unusual numbers of the 

 goshawk and snowy owl, as these seem to come dur- 

 ing years when disease has destroyed the northern 

 hares that form their usual food supply. On these 

 occasions, Indians, lynxes, and other hunting mam- 

 mals are often hard put for food, while the hunting 

 birds exercise their powers of flight to carry them to 

 other regions where food may be procured. 



Less certainly has the coming of crossbills been 

 attributed to failure of their normal food supply of 

 cones in the north — a factor that may perhaps 

 operate with the thick-billed parrot, which also feeds 

 on the seeds of pines as well as on acorns. 



Of interest in this connection are the movements 

 of the sand-grouse of Eurasia, curious birds with 

 many attributes of grouse and shore-birds, that 

 seem anatomically most closely allied to the pigeons. 

 Sand-grouse are regular inhabitants of the high 

 plateaus or steppes of central Asia, but may range in 

 great flocks through the surrounding territory. 

 Henke found them in 1876 breeding in numbers on 

 the Kirghiz steppes, where nomadic tribes told him 

 they had been unknown before. In the spring and 

 summer of 1863 a flight of these birds spread across 

 western Europe, reaching the coast of Donegal in 



