50 The Irish Nahiralist. [Jan., 



it is a matter of indifference to the migrants ; but if more 

 pronouncedly so, their movements are slightly quickened 

 thereby. The duration of such favourable spells is sooner 

 or later broken by the advent of a cyclonic period, which 

 interferes, to a greater or lesser degree, with the progress of 

 migratory movements. Unfavourable weather-conditions of 

 a pronounced nature temporarily interrupt the ordinary 

 movements. The weather incentives to migration are of 

 different kinds. First, there may be favourable weather- 

 periods immediately following unfavourable periods. 

 Secondl3% they ma}^ be due to unfavourable weather, such as 

 lower temperature, which either compels the birds to move, 

 or acts as a warning to them to do so. Thirdly, and on the 

 other hand, genial spring weather is an incentive to a north- 

 ward move. Temperature plays the most important part in 

 the various seasonal movements. 



All the great autumnal immigrations coincide with favour- 

 able weather-conditions in north-western Europe — namely, 

 the presence of a large well-defined anticyclone over 

 Scandinavia, with gentle gradients to the south-west. On the 

 other hand, cyclonic conditions may prevail west of the British 

 area, with a low pressure centre off the west of Ireland. Under 

 these conditions the weather is clear and cold, with light 

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

 sky is overcast, with easterly winds, and frequently with fog 

 on the east coast. These conditions usually follow the pass- 

 ing away of a cyclonic spell from Scandinavia, during which 

 ordinary migratory movements are interrupted. Movements 

 from the east by the East and West Route are most pronounced 

 during similar favourable weather-conditions. All the 

 autumn movements are stimulated by a fall in the tempera- 

 ture. In connection with spring immigration several 

 unusually early appearances have been recorded, and the 

 daily weather report shows that the localities where these 

 early occurrences took place were at the time the warmest 

 spots in Western Europe. 



A careful comparison made between the migrational and 

 meteorological phenomena in connection with these move- 

 ments from the Continent shows that all such movements, 

 except those performed late in the season, are to be correlated 



