88 A HISTORY OF BIRDS 



Storms and droughts, and cataclysmal disturbance, how- 

 ever, though they very materially affect bird-life, do so for the 

 most part only indirectly. That is to say, the shifting from 

 one area to another, and the mortality to which reference has 

 been made, are not so much due to these adversities as to the 

 lack of food or the inability to procure it. 



Since birds neither aestivate nor hybernate — though the 

 older naturalists were convinced that the Swallows and some 

 other birds really did hybernate — such species as cannot obtain 

 a sufficiency must wander till they find this. With some 

 species such wanderings are limited to a relatively small area, 

 but with others vast distances are annually traversed on this 

 account : though there are many cases, to which reference will 

 be made in due course, which seem to be prompted by some 

 other motive. 



These periodic movements constitute the study of migration. 



