68 



" The weather incentives to migration are widely different in 

 their nature, and may take several forms. First, there may be 

 favourable weather periods immediately following unfavourable 

 periods. Secondly, they may be due to weather in certain 

 respects unfavourable to the birds, such as a decided fall in 

 temperature, which either compels birds to move, or acts as a 

 w^arning that the time has arrived for their departure south- 

 wards. Such cold spells are characteristic of anti-cyclonic 

 periods, when the weather is calm and highly favourable for a 

 prolonged flight. Thirdly, and on the other hand, the advent in 

 spring of a genial spell, especially if accompanied by a rise of 

 temperature, is an incentive to a move to the northward for the 

 summer haunts. 



" The weather influences thus vary considerably ; but tempera- 

 ture plays the most important part in the various seasonal 

 movements, and is the main controlling factor in all extraordin- 

 ary movements, other meteorological conditions being suitable. 

 Each movement, however, has its peculiarity, and the conditions 

 controlling it are often due to meteorological phenomena of a more 

 or less complex nature, most of which, perhaps, admit of 

 explanation." 



Eef erring to the great autumnal movements of birds to 

 our eastern shores, Mr. Eagle Clark remarks, p. 20 — "It has 

 been ascertained that all these great movements are due to 

 the prevalence in north-western Europe of weather condi- 

 tions favourable for migration. These conditions are the result 

 of the following type of pressure distribution, namely, the 

 presence of a large and well defined anti-cyclone over the 

 Scandinavian Peninsula, with gentle gradients extending in a 

 south-westerly direction over the North Sea. On the other hand, 

 cyclonic conditions prevail to the westward of the British area, 

 with a low pressure centre off the west coast of Ireland, or, 

 though less frequently, over areas further south. Under these 

 pressure conditions the weather is clear and cold, with light 

 variable airs over Norway and Sweden, while in Britain the sky 

 is overcast, and moderate and strong easterly winds are ex- 

 perienced, with fog not unfrequently prevailing at many east 

 coast stations. 



