186 STUDIES IN BIRD-MIGRATION 



conditions favourable for them prevail over the British 

 area and to the southwards (see Weather Charts). 

 Indeed, the movements are sometimes kept quite dis- 

 tinct from immigrations by the interposition of weather 

 barriers to the north, which cut off migratory com- 

 rpunication between our shores and those of Northern 

 Europe. These barriers most frequently take the form 

 of a subsidiary low-pressure area lying over the North 

 Sea between Great Britain and Scandinavia. 



The great emigrations from Britain and Ireland, like 

 those from Northern Europe at the same season, set in 

 on the passing away of cyclonic conditions. The un- 

 favourable conditions which have passed away have 

 probably acted as a warning to many laggard migratory 

 birds, while the cold adds an additional spur and swells 

 the ranks of the departing birds. 



During October local movements are observed, 

 which are directly traceable to the influence exerted on 

 emigration by a considerable lowering of the tempera- 

 ture over a particular area. Thus, for example, on 

 20th October 1883, there was a remarkable movement 

 of Swallows to the south-east coast of Ireland. On this 

 day there was a decided fall in temperature, the lowest 

 readings being recorded for Ireland, where these laggard 

 summer-birds had until then found congenial quarters. 

 Ao-ain, on loth October 1885, a movement to the 

 southward of Thrushes and Blackbirds was recorded 

 at stations in the north of Scotland, and in this 

 instance, too, the meteorological data afford the infor- 

 mation that a fall in temperature had occurred within 

 that area. 



The emigratory movements of winter are, as has 

 been already stated (p. t^j), attributable to the direct 



