SLWJ/ARV AXD CONCLUSION 291 



Having shown the origin of these Routes of Migration, 

 we next proceed to discuss the Conditions of Fh'ght. 

 Dealing first with the Instinctive impulse of Migration, 

 we then pass to the question of how birds are able to 

 traverse these routes with such apparent precision. 

 Experience rather than Inherited Impulse is shown to 

 be the guiding influence. The altitude of migration 

 flight, and the order of migration are next discussed. 

 The daily time of migration, and the gregariousness or 

 otherwise of birds on passage are then described ; whilst 

 finally the perils of migration are briefly treated. 



Our next two chapters are devoted to a general 

 description of the Spring and Autumn Aspects of 

 Migration in the British Area. We have traced the 

 phenomenon in spring from its earliest beginning in the 

 departure of those migrants to the east across the North 

 Sea that breed in continental areas, and the almost 

 simultaneous arrival of other migrants, many of the 

 same species from more southern areas, that breed in 

 Britain. Later we describe the departure north or 

 north-east of our winter visitors ; together with the vast 

 amount of coasting migration that passes over the 

 British Area, composed of birds that winter to the south 

 of us and breed to the north. Coming then to the 

 arrival of spring migrants to the British Islands, we. trace 

 the movement from its beginning onwards through the 

 months that it continues, until it finally dies away for 

 the season. A few remarks on the vertical migration 

 of birds in spring, species that ascend to various eleva- 

 tions for the purpose of breeding, together with a table 

 indicating the duration of Flight, bring the spring 

 aspects of the phenomenon to a close. Entering then 



