10 THE BIOLOGY OF BIRDS 



§ 9. Migration 



Finally it seems permissible to link on to the unified 

 harmony of bird characters the very widely expressed 

 custom of migrating. Given the power of flight, which 

 means rapid escape from areas where food and drink have 

 become scarce, and where other conditions, such as tempera- 

 ture, occasion discomfort, the possibility is opened up of 

 circumventing the difficulties of the seasons. From the 

 summer nesting quarters, when scarcity and cold set in, 

 the migrants fly to easier winter quarters where they can 

 rest and recuperate. From the winter quarters, when these 

 become too warm for safe brooding and too dry for the 

 bird's thirst, the migrants return to the country of their 

 birth. The details do not concern us now, but it does not 

 seem far-fetched to say that the migratory custom, which is 

 now an organic impulse in a large proportion of birds, is 

 congruent with the salient characteristics of the class. 



The object of this introductory chapter is twofold : 

 first, to suggest that many of the salient characteristics of 

 birds are congruent or correlated ; and second, to think 

 back to the deep constitutional characteristics which may 

 have been the pre-conditions of flight. These deep 

 characters are intensity of metabolism, spare muscular 

 habit of body, rapid bipedal progression, the supply of the 

 body with arterial blood exclusively and that of fine quality, 

 warm-bloodedness, the assisting of respiration by locomotion, 

 and a well-developed brain. Given these features and 

 others, it seems easier to understand how the evolution of 

 feathers would make flight possible. And then would 

 follow a score of secondary adaptations to flight. 



