THE MIGRATION OF BIRDS 



There is a theory held by many naturalists that 

 the migrating instinct dates back to the glacial 

 period. According to this theory North America 

 was inhabited originally by non-migrating birds. 

 Then the great Arctic ice-cap began to move south- 

 ward and the birds were forced to flee before it or 

 starve. Now and then during the subsequent period 

 the ice receded and the birds returned, only to be 

 driven again before the next onrush of the Ice King. 

 Thus during these centuries of alternate advance 

 and retreat of the continental glacier, the birds ac- 

 quired a habit, which later became an instinct, of 

 retreating southward upon the approach of cold 

 weather and coming back again when the ice and 

 snow showed indications of passing away. 



The Gathering Flocks. — To the bird student there is 

 keen delight in watching for the first spring arrivals 

 and noting their departure with the dying year. It 

 is usually in August that we first observe an un- 

 wonted restlessness on the part of our birds which 

 tells us that they have begun to hear the call of the 

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