12 Introduction 



Antarctic climates. All birds that fly are capable of 

 migration to a greater or less extent, but aU do not 

 take full advantage of their capabilities. 



The rettirn migration in spring to the northerly 

 breeding haimts may be due to some hereditary instinct 

 — ^whatever that expression may mean — or to the fact 

 that the birds seek what they know to be the places 

 most favourable for rearing their young in the particular 

 manner to which they are accustomed, or even to the 

 pressure of the species of the southern hemisphere 

 which may be returning from their winter quarters, 

 though this argument does not seem very convincing. 



It has also been suggested that migration may be 

 due to the same cause as an extension of breeding 

 range ; that is, the numbers may become too great to 

 be contained in the original summer haunts, and under 

 pressure a certain proportion may move further to the 

 south, when they cannot well go further northward. 

 But, since it can hardly be contended that there is 

 absolutely no room left in most areas for the birds' nests, 

 and siace so many of them breed in colonies and 

 do not at present dispute the possession of every inch 

 of soil, this argument practically resolves itself into 

 that concerned with scarcity of food. 



Finally it may be observed that the length of the 

 journeys taken by various species of birds differs 

 immensely j many move to comparatively short dis- 

 tances, while such forms as the Turnstone and the 

 Sanderling, which breed in the far north, go so far 

 south in winter that they may practically be con- 

 sidered cosmopohtan. 



Migration naturally leads to thoughts of geographical 

 distribution, and care must be taken not to confound 



