172 STUDIES IN BIRD-MIGRATION 



migration, it is essential that the weather conditions 

 prevailing in the area in which the movement had its 

 origin should be taken into consideration. Here alone, 

 as a rule, has the weather any bearing upon the move- 

 ment under investigation ; the weather prevailing in 

 places reached after a more or less extended flight is 

 nothing to the purpose. Thus we must look to Con- 

 tinental conditions for an explanation of the arrival of 

 immigrants on our shores, in both spring and autumn ; 

 and to our home weather for the key to the cause of the 

 departure of emigrants from the British area at all 

 seasons. 



As the result of much careful study of a vast amount 

 of material, migrational and meteorological, some very 

 important facts have been made abundantly manifest, 

 foremost among which are the following : — 



(i) That the type of weather which prevails when 

 the winds range from south to east, and especially when 

 they are south-east, is the most favoitrable for migratory 

 movements between the British Islands and Continental 

 Europe, in both spring and autumn. (2) That the 

 weather conditions prevailing when the winds vary from 

 west to north are the most unfavourable, both for immi- 

 gration to and emio^ration from the British Isles. 



Year after year, the accuracy of these findings has 

 been repeatedly impressed upon me during my practical 

 work at widely separated observing-stations both at 

 home and abroad ; and they have also become known 

 to a number of observers (lighthouse-keepers and others) 

 who have assisted me in the investigations. 



With these well-ascertained facts as a basis to work 

 upon, let us proceed to inquire what the types of weather 

 are which have such remarkable influences over the 



