MIGRATION AND WEATHER 87 



whole of the British Islands, simultaneous immigra- 

 tion and emigration may be witnessed. 



Cyclonic spells are not always unfavourable to 

 migration. In spring, when they are of a mild 

 type with soft rain and warm winds following after 

 a cold anticyclonic period, a northward movement 

 is frequent. 



Mr Eagle Clarke says that the importance of 

 winds is overstated, but as an incentive only. The 

 direction of the wind has no influence as an in- 

 centive but its force is an important factor ; in a 

 strong wind a bird may be blown out of its course. 

 Birds will not start in a high wind but may pass 

 into the influence of strong winds which may affect 

 both progress and direction. He adds that par- 

 ticular winds usually prevail during the season of 

 great autumn movements, which are not incentives 

 but are the result of pressure distribution which is 

 favourable to migration. These are usually north- 

 east to south, but a westerly wind would serve as 

 well, but it indicates a pressure distribution which 

 is fatal to migration between north-west Europe 

 and Britain cyclonic areas to the north-east and 

 east of our area. 



All this, no doubt, is perfectly true. It is founded 

 on the analysis of a huge number of carefully re- 

 corded observations, and upon a general know- 

 ledge of migration which few can ever hope to 



