NOMADIC MIGRATION. 159 



find the greatest amount of Nomadic Migration 

 ])revairmg among birds peculiar to the coldest 

 regions of the earth, either on mountains or in liio-h 

 northern latitudes — species whose supply of food 

 is not curtailed by any decrease of temperature, 

 able to live throughout the long Arctic winter 

 wherever the snow does not absolutely cover the 

 various substances on which they live. 



These nomadic migrants very forcibly show 

 that Want of Food was one of the great initiating 

 causes of regular migration in autumn, just as High 

 Temperature was probably the great initiating cause 

 in spring. So long as food can be obtained, most 

 birds show great reluctance to adopt a migratory 

 habit in autumn; therefore throughout the Arctic 

 regions, except, perhaps in the Polar zone, some 

 birds may be found all the winter through wherever 

 food can be obtained. No insectivorous birds are 

 known to winter above the isothermal line of pre- 

 vailing snow and frost at that season, but manv 

 species of birds whose food consists of buds, twio-s, 

 seeds, and berries ; or that subsist on any carrion 

 or refuse ; or that prey upon these other birds 

 themselves, habitually remain near this snow-clad 

 and frost-bound area, moving about just as the food 

 supply may fluctuate, sometimes wandering south 

 during a spell of unfavourable weather, but hasten- 

 ing north again as easier climatal conditions recur. 



Of course the birds that habitually winter in the 

 Arctic regions are comparatively few, for the simple 

 reason that the great majority of species that visit 



