18 BIRD-LIFE OF THE BORDERS 



would suggest beginning with the belief that every 

 individual bird one may see in garden or wood, on fell 

 or shore, has travelled to Africa and back — or alterna- 

 tively, to the Arctic and back- — since you saw it a year 

 ago. This applies to the thrush that is nesting in your 

 apple-tree, and the starling that wakes you at four o'clock 

 on spring mornings with its squalling brood under your 

 eaves. Then, throw the burden of disproof upon that 

 dozen or so of absolutely stationary species, a list of which 

 I have just promised to insert later in this book. 



Now, why do birds migrate ? The question, at first 

 sight, appears a simple one, and several answers at once 

 suggest themselves. In reality, however, it is a many- 

 sided biological problem, and one of no small complexity 

 and mystery. 



Suitable climatic conditions and temperature, food- 

 requirements and distribution in proportion to food- 

 supply, are among the more obvious answers to the 

 question. These, and similar circumstances, influence, 

 and to some extent regulate, migration ; but, on exam- 

 ining more deeply into the subject, it becomes clear that 

 though they may form regulating factors in migration, 

 yet they are not its primary cause. 



Thus, with regard to food-requirements, it is obvious 

 that when birds of a given species are found permanently 

 inhabiting a certain area at all seasons, the natural or 

 climatic conditions of that area do not render migration 

 imperative. Therefore, when it is seen that large bodies 

 of such birds do migrate and traverse perhaps great 

 distances, it is clear that any hypotheses based on 

 considerations of temperature, climatic conditions, or 

 the like, must be abandoned. Such movements, it is 

 possible, may be dictated by the quantity (as opposed 



