CHAPTER VII 



MIGRATION AND WEATHER 



IN previous chapters it has been necessary to refer 

 repeatedly to the connection between migration 

 and meteorology ; either the relation of periodic 

 movements to the rotation of seasons, or the influence 

 directly or indirectly of weather conditions upon 

 normal and . abnormal migration. That there is 

 an overruling relation between the advance of 

 spring and the passage to northern breeding quarters, 

 and the gradual cooling in autumn and the retreat 

 to winter quarters is, of course, evident, but it must 

 not be held, as contended by the early students of 

 migration, that this is the sole factor which regulates 

 migration. The actual relationship between the 

 weather and the movement of birds is far more 

 complicated than one. would imagine, and the 

 stimuli of continental or overland travelling differ 

 from those of a cross-sea flight. 



In the British Islands most of our larger move- 

 ments are at their start or their finish, or both (so 

 far as our area is concerned), oversea passages, and 

 unless the weather be absolutely favourable, birds ' 

 do not undertake these voyages. No one has 



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