86 THE MIGRATION OF BIRDS 



a rush ; a sudden fall in temperature may force 

 large numbers of birds on in autumn or retard them 

 in spring. Temperature, he declares, is the main 

 controlling factor in all extraordinary movements, 

 other meteorological conditions being suitable. 



In the autumn migration to Britain, the chief 

 movements take place when a large and well- 

 defined anticyclone has its centre somewhere over 

 Scandinavia, with gentle gradients in a south- 

 westerly direction over the North Sea. Coincident 

 with this we usually find cyclonic conditions pre- 

 vailing to the west of the British area, with low- 

 pressure centres off the west or south-west of 

 Ireland. The weather is clear and cold, with light 

 variable airs over Scandinavia, but in Britain the 

 sky is overcast, and the wind easterly and moderate 

 to strong ; not infrequently these conditions mean 

 fog on our eastern coasts. If the birds leave Scandi- 

 navia under favourable conditions they may be 

 met by the approaching cyclonic system, which 

 usually, though by no means always, travels in 

 a north-easterly direction across the Atlantic. 

 Migration is thus checked, but a return of favourable 

 anticyclonic conditions starts the birds again, often 

 with a fresh impulse in the shape of falling temper- 

 ature. When the anticyclonic area is exceptionally 

 large, extending from the Scandinavian peninsula 

 in a south-westerly direction and embracing the 



