24 THE MIGRATION OF BIRDS 



southward is surely due to the fact that the tem- 

 perature is falling in the north more rapidly than 

 in the south. That they are not always due south 

 is certain. The American golden plover, as Mr 

 Wells W. Cooke so lucidly demonstrates, at first 

 travels eastwards from its home in western Arctic 

 America to the fruit-laden lands of Labrador and 

 Nova Scotia, where it feeds for some time, stoking 

 up for its long over-sea journey due south. Mr 

 Cooke says, " It can also be said that food supplies 

 en route have been the determining factor in the 

 choice of one course in preference to another, .and 

 not the distance from one food base to the next. 

 The location of plenty of suitable provender having 

 been ascertained, the birds pay no attention to the 

 length of the single flight required to reach it " (21). 

 During the evolution of the route many bases would 

 be found which were superior to others, and skipping 

 and the gradual shortening of the journey from one 

 to another would result. The final goal, the food 

 base which in any weather or season provides the 

 safe sufficiency of food, having been reached by 

 the birds, this becomes the winter quarters. They 

 return to this secure retreat each winter, instead 

 of aimlessly wandering in search of a better, and 

 thus the long-distance migratory habit is formed. 

 Heredity tends to confirm this and it becomes an 

 instinct, 



