CHAPTER II 



CAUSE AND ORIGIN OF MIGRATION 



THE question What makes Birds Migrate ? or 

 what causes them to remove from one zone to 

 another at certain seasons, has been answered, no 

 doubt to the satisfaction of the respondents, in 

 many varied ways. Closely connected with the 

 question of immediate impulse is the deeper, and 

 less easy to prove problem as to how migration 

 originated. 



It has been dogmatically asserted repeatedly 

 that birds invariably breed in the most northerly 

 part of their range, and winter in the most southerly. 

 Winter, when speaking of Holarctic birds, only 

 applies to the season in the northern hemisphere ; 

 the birds which pass south of the equator winter in 

 summer. Whilst accepting this as a rule, two 

 reservations must be made. First, that it only 

 applies to birds of the northern hemisphere, and 

 secondly that it is a rule with exceptions. It seems 

 probable that the breeding area of some of the birds 

 which reach the British Islands in autumn by the 

 so-called east and west route is in more southerly 

 latitudes than our islands, and certainly it seems 



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