2 THE MIGRATION OF BIRDS 



away with the need of definition, especially as 

 there are many complicated phases of migration. 

 The migration of birds is as a rule between the 

 breeding area or home and the winter quarters, 

 but there are many migrants which never reach 

 breeding quarters in spring, and many others which 

 leave the regular breeding quarters or the place of 

 residence in winter to perform a very real migration 

 under peculiar stress of circumstances. Again the 

 spasmodic movements of certain gregarious species, 

 which at irregular intervals change their location 

 in large numbers to take up their abode in another 

 part of the range, is really migration, though it is 

 now usually described as irruption, incursion or 

 invasion. 



Newton says (38) that bird migration is " most 

 strangely and unaccountably confounded by many 

 writers with the subject of Distribution," but the 

 very act of the bird which extends its range, the 

 first step in distribution, is migration. The histories 

 of present-day distribution and migration are 

 irrevocably interwoven ; as Mr P. A. Taverner 

 remarks (51), " migration is a dispersal, and con- 

 versely, this dispersal, as it manifests itself, is 

 migration," whilst distribution is the outcome of 

 dispersal. 



Broadly speaking, all birds migrate, though the 

 length of the journey varies in different species, 



