108 TEE LIFE OF THE FIELDS. 



seek a warmer climate and more to eat; more par- 

 ticularly probably for sustenance. 



The original and simple theory that the majority of 

 birds migrate for food or warmth is not overthrown 

 by modern observations. That appears to be the 

 primary impulse, though others may be traced or 

 reasonably imagined. To suppose, as has been put 

 forward, that birds are endowed with a migratory 

 instinct for the express purpose of keeping down 

 their numbers, in order, that is, that they may perish 

 in crossing the sea, is really too absurd for serious 

 consideration. If that were the end in view, it would 

 be most easily obtained by keeping them at home, 

 where snow would speedily starve them. On the 

 contrary, it will appear to any one who walks about 

 woods and fields that migration is essential to the 

 preservation of these creatures. By migration, in 

 fact, the species is kept in existence, and room is 

 found for life. Apart from the necessity of food, 

 movement and change is one of the most powerful 

 agencies in renewing health. This we see in our 

 own experience ; the condition of the air is especially 

 important, and it is well within reasonable supposi- 

 tion that some birds and animals may wish to avoid 

 certain states of atmosphere. There is, too, the 

 question of moulting and change of plumage, and the 

 possibility that this physiological event may influence 

 the removal to a different climate. Birds migrate 

 principally for food and warmth ; secondly, on account 

 of the pressure of numbers (for in good seasons they 

 increase very fast) ; thirdly, for the sake of health ; 

 fourthly, for sexual reasons ; fifthly, from the operation 



