WEATHER INFLUENCES 173 



migrants or would-be migrants. It must first be re- 

 marked, however, that the direction of the wind has 

 in itself nothing to do with the results described. The 

 winds and the performance, or non-performance, of the 

 migratory movements are the effects of a common 

 cause — namely, the particular type of weather prevailing 

 at the time, which may be favourable or unfavourable 

 for the (light of birds to or from our islands. 



Favourable Weather Conditions. — The favourable 

 conditions prevail when a large anticyclone has its 

 centre in North - Western Europe, covers the North 

 Sea, and extends to the British Isles. This would 

 give light gradients (isobars far apart), gentle winds, 

 and, generally speaking, fine weather between the British 

 Isles and Scandinavia. Such a weather period would 

 be eminently favourable for the passage of birds along 

 one of the most important lines of flight for migrants 

 between the Continent and our islands. On these 

 occasions, too, the barometric pressure being highest to 

 the east of our islands, the wind, according to general 

 rule, would be between south and east over the western 

 shores of the North Sea (see Plate V.). 



When such weather conditions prevail in the autumn, 

 much emigration takes place from North- Western Europe 

 towards the British coasts, where the migrants make 

 their appearance in great numbers. When the same 

 type of weather obtains in the spring, great emigrations 

 from our shores set in towards the north and east, of de- 

 parting winter guests, and birds of passage, returning to 

 their summer quarters. The pulse, if one may be 

 allowed the term, of the movements is quickened from 

 time to time through the stimulating influence of 

 temperature — a fall in autumn, a rise in spring. 



