26 THE MIGRATIONS OF BIRDS 



gards life in equatorial regions, where the hours of 

 the day remain evenly divided between daylight and 

 darkness, and where the favorable period of night is 

 not curtailed as it is in June in the North Temperate 

 Zone. 



Among other theories, it has been suggested that 

 migration is the natural outcome of power of flight, 

 a faculty that is so pleasurable and so easy in execu- 

 tion that birds migrate naturally from place to 

 place when not bound by home ties. It has been 

 asserted also that migration has its impetus in a 

 desire for solitude, which causes birds to seek remote 

 islands, isolated lakes, marshes, or distant forests, 

 where they may be free from all disturbing factors 

 during the period of reproduction. Though many 

 birds, particularly those of shy habit, do seek isola- 

 tion while breeding, there are others with exactly 

 opposite tendency, as the many birds that nest 

 familiarly about the homes of man; so that there 

 are as many points against this argument as for it. 



Dr. Alvin R. Cahn, in a recent article ' on the 

 migration of animals, has discussed migratory 

 movements in a highly interesting and instructive 

 manner. He points out that the movements of birds 

 in north and south migration correspond, in general, 

 to advance and retreat to or from the breeding- 

 station, and considers migration due to physiological 



^ American Naturalist (1925), lix, 539-566, 



