CAUSE AND ORIGIN OF MIGRATION 15 



root of the migration of birds. The advantages 

 of the power of flight, to which also it owes its 

 development, include the ability to avoid active 

 and passive enemies, and to remove from one feeding 

 ground to another undeterred by the barriers which 

 restrict the terrestrial animal. A natural sequence 

 of this ability to take advantage of aerial locomotion 

 is the habit of wandering in search of food, more or 

 less noticeable in all birds. The habit of wandering 

 led to the discovery of feeding grounds and suitable 

 nesting places ; where these nesting places, probably 

 at first, only removed a short distance from the 

 parents' nesting site, were suitable, dispersal and 

 an extension of the distributional area or range 

 of the species followed ; but where the feeding 

 area was unsuited or not so well suited to the needs 

 of the species, hereditary attachment to the original 

 home and memory of the direction of this home, 

 or even in some cases accidental wandering back to 

 the more suitable locality, would originate a migra- 

 tion. Coupled with this are two important factors 

 which would tend to make the habit periodical 

 and regular both as regards time and locality. 

 The memory of the bird, call it instinctive memory 

 if we like, would limit the wanderings in search of 

 food to a certain number of places where food was 

 most abundantly found, and the passage between 

 feeding area and breeding area become regular 



