Natural History 429 



many birds on the approach of winter we can perhaps see good 

 reason, probably not so much in mere cold itself, but in the de- 

 crease in food-supply, in the freezing of ground and water, and 

 in the shortened hours of daylight. In the return from the south 

 in the spring we may see an expression of a need for expansion 

 during the breeding season to obtain more room, abundance of 

 nesting-sites, and fresh sources of food-supply. 



We must distinguish carefully between reasons and causes of 

 migration between "why?" and "how?" Valid although the 

 reasons given may be, they do not in the least explain how the 

 migratory habit has come to be; to miss this point is to fall into 

 the trap of imagining birds as endowed with human knowledge 

 and intellect with the power of adopting a reasoned course of 

 conduct, based on the foreknowledge of seasonal events and on 

 an appreciation of geographical differences. 



Causes of Migration 



Two points must strike us as being significant. One is that 

 migration is a very regular phenomenon, happening year after 

 year according to the same pattern, without marked differences 

 corresponding to annual variations in climate, and showing none 

 of the features to be expected in an emergency effort created anew 

 each season. Secondly, much migration takes place long before it 

 seems to be necessary, for in the British Isles southward move- 

 ments begin as early as July. Many migrants, too, go further 

 than seems to be required, overshooting the mild winter of the 

 northern subtropics to find a similar climate in the summer of 

 temperate regions of the Southern Hemisphere. 



The conclusion seems inevitable that migration is a very old 

 habit, an inborn instinct which was developed ages ago, and which 

 manifests itself year after year in a uniform manner and without 

 any remarkably close conformity to immediate conditions. For an 

 explanation of this ancient origin of the instinct we should doubt- 

 less look to the former history of birds for some more compelling 



