184 STUDIES IN BIRD-MIGRATION 



associated with the great weather changes of the 

 autumn. 



It has been ascertained then that all these great 

 movements are due to weather conditions which have 

 been already described as being favourable — namely, 

 the prevalence of fine weather between Scandinavia and 

 Britain. These conditions often follow the passing 

 away from North- Western Europe of a cyclonic spell of 

 a more or less pronounced nature, during the prevalence 

 of which the ordinary course of the emigratory move- 

 ments is either interrupted or rendered impossible. 

 The anticyclone removes the cyclonic weather barrier, 

 releases the pent-up migrants, and provides condi- 

 tions favourable for migration, sometimes adding 

 also an incentive in the form of a decided fall in 

 temperature. 



Autumn Emigration. — The chief feature in migration 

 during the earlier autumn days is the departure of 

 British summer birds, including those which have been 

 described as partial migrants. July, in some seasons, 

 has its ungenial spells, and these make themselves 

 felt by our feathered guests, resulting in movements 

 of a partial or a more decided nature. The influences 

 inciting these incipient movements are a complete 

 break-up of normal summer weather and the prevalence 

 of unsettled conditions, not unfrequently accompanied 

 by thunder and heavy rains and a decided fall in 

 temperature. The result upon our summer visitants, 

 or upon their young, on such occasions, is that many 

 of them move from their accustomed haunts, and appear 

 on the coast — sometimes at the lanterns — where the 

 occurrence of those departing from our shores is duly 

 chronicled. The species chiefly affected arc the Thrush, 



