iS97-] iPALMKR — The Migratio7i of Birds. 2t 



with a rise of temperature in south-west Europe, which evi- 

 dently induces the birds to start northwards. In not a few 

 instances such movements are recorded for dates on which 

 the temperature in Britain was lower than immediately before 

 the immigration. This indicates that the increase of warmth 

 at the seat of emigration is the main factor influencing the 

 northward spring movement. This rise of temperature 

 sometimes extends over the British Isles. Apart from this 

 simple phenomenon no other peculiar meteorological condition 

 appears to be associated with these spring movements. The 

 spring migration from our islands to northern breeding- 

 grounds is influenced by the weather-conditions prevailing at 

 the time in the British area. This emigration, however, 

 naturally takes place later than the corresponding movement 

 into Britain from the south : it appears to commence in April 

 and continues during May. It is embarked upon under the 

 same type of pressure-distribution as that which is favourable 

 to the autumn migration, namely, a high pressure centre over 

 Norway and Sweden, with gentle gradients to the south-west. 

 Under these circumstances there is fine weather over the 

 North Sea. 



The anticyclonic, or fine-weather periods in April are 

 favourable to migration if the temperature is fairly high. 

 Cyclonic periods as a rule are unfavourable owning to their 

 high winds and ungeniality; on the other hand, when they are 

 mild and follow a cold spell they are favourable to a north- 

 ward migration from Britain. In autumn marine species, such 

 as Skuas, Petrels, Phalaropes, &c., are occasionally driven 

 out of their course by gales, when the appear on our coasts 

 in large numbers, and are sometimes driven inland. 



A careful study of the subject shows that the direction of 

 the wind has no influence as an incentive to migration, but 

 that its force is an important factor. The birds do not appear to 

 be concerned by ordinary winds, but they do not migrate when 

 the winds are exceptionally high. Particular winds usually 

 prevail during the great autumnal movements. These, 

 although favourable, are not an incentive to migration, 

 but are the winds that accompany the high pressure centre 

 over Norway, already referred to. 



