52 Twelve Months With 



Many influences, however, have a more or less 

 important bearing upon the migration of the 

 birds, among which may be mentioned coast lines, 

 river channels, food supply, sex impulses, hun- 

 ger, love, homing instinct, inherited or acquirjed 

 memory, temperature, storm conditions, magnetic 

 meridians, etc., all of which are good as far as 

 they go, but none of them, nor all of them together 

 can wholly account for the phenomenon. 



The ancients observed that some birds visited 

 them only for short periods and at certain seasons, 

 but they apparently sought or found no explana- 

 tion for it, and some thought that the birds 

 hibernated, after the manner of certain animals 

 who thus survive the period when the food 

 supply is entirely cut off. 



While flying is not the only way in which 

 animals migrate, it is the most effective, and most 

 of the birds are thus structurally well provided 

 with the means of escaping from the disastrous 

 effects of adverse circumstances, and in this way 

 nature has wisely provided against the necessity 

 for hibernation. 



The fact that the birds are endowed with the 

 power of flight suggests some things which doubt- 

 less have had an influence upon the cause or 

 origin of migration. For example, this power 

 enables the birds to avoid many of their natural 

 enemies, and also to move rapidly from one feed- 

 ing ground to another. They have therefore 

 naturally traveled away from those things which 



