WEATHER INFLITENCES 183 



or they may be distinctly unfavourable through being 

 decidedly cold. Some emigration, of a straggling nature, 

 it is true, is recorded during anticyclonic periods when 

 moderately cold but calm weather prevails. 



In spring, cyclonic periods, too, vary in their 

 influences on emigration. They are, as a rule, un- 

 favourable owing to their high winds and ungeniality. 

 On the other hand, when they are of a mild type, and 

 characterised by warm rain and soft breezes, following a 

 cold anticyclonic spell in April, they favour to a north- 

 ward movement from our islands. 



The spring emigrations are embarked upon under 

 precisely the same type of pressure distribution as that 

 described as being favourable for the autumnal passage 

 of birds across the North Sea to our islands — namely, 

 when an anticyclonic area extends from north-western 

 Europe to the shores of Britain. Under such meteoro- 

 logical conditions, the North Sea is spanned by fine 

 weather, and moderate easterly to southerly breezes 

 prevail. Some of these spring movements to the north 

 are occasionally undertaken during somewhat unfavour- 

 able weather. Even in May there are records of 

 emigration during sleet, cold rain, and north-east 

 breezes, but it has to be explained that these conditions 

 followed prolonged spells of inclement weather, and 

 were genial in comparison with them. 



Autumn Immigration. — The arrival movements of 

 birds in the British Isles during the early autumn are 

 undertaken under weather conditions which were favour- 

 able for their performance. 



It is not until the latter half of September, and 

 during October and early November, that the move- 

 ments into our islands from the north-east are to be 



