THEORIES OF MIGRATION 21 



hibernation, which has been alleged sporadically to 

 account for various circumstances at the time not 

 fully understood. 



Scientific Hypotheses of Migration 



With the migration of birds recognized as a regu- 

 lar accompaniment of the changing seasons, there 

 arises for consideration the manner in which migra- 

 tion had its origin, and the reasons that lie behind 

 its continuation — matters still of considerable 

 dispute. The various theories that have been ad- 

 vanced may be outlined briefly. 



The most commonly accepted explanation of the 

 seasonal movement of birds is that of shifting food- 

 supply. Failure of food in northern areas in autumn 

 forces certain birds to travel; it is only in a southerly 

 direction that amelioration of the unfavorable con- 

 dition is encountered, so that the birds affected move 

 south to an area where food is abundant. With the 

 coming of spring the need for reproduction to per- 

 petuate the species becomes paramount. The food- 

 supply of the southern regions to which these birds 

 have migrated does not permit the easy support of 

 the host of young that must be reared to perpetuate 

 each species, so that the individuals that have come 

 from the north press out in that direction until they 

 arrive at a suitable spot for settlement for the breed- 

 ing season. 



