180 STUDIES IN BIRD-MIGRATION 



to migrate, through its effect upon their food suppHes. 

 Should the weather be unfavourable for a prolonged 

 flio-ht, the would-be emigrants have to await the 

 advent of propitious conditions, and then a rush to the 

 south or west, as the case may be, follows. Cold 

 spells, however, are not unfrequently associated with 

 anticyclonic periods, during which, too, the weather is 

 calm, and hence suitable for departure movements from 

 the area over which they prevail. 



Fog. — It often happens that during an important 

 migratory movement in the autumn fog prevails. On 

 such occasions more birds than usual approach the 

 lanterns of the light-stations, where they are sometimes 

 killed in considerable numbers. This phenomenon is 

 another effect of those anticyclonic conditions which 

 have been alluded to as favourable for migration, and it 

 is thus not surprising that the birds should encounter 

 foggy weather during their great movements. There is 

 also some direct evidence that birds lose themselves in 

 foggy weather, since practically non-migratory species 

 appear during its prevalence at unusual seasons at 

 stations just off the coast. 



Dr Shaw informs me that in the past few years he 

 has had a large correspondence, chiefly through Dr 

 Tressider, with various pigeon-flying associations in 

 connection with pigeon-matches. This has led him 

 to conclude that a successful pigeon-race requires anti- 

 cyclonic conditions, so much so that the custodian of 

 many thousands of pigeons sometimes waits in France 

 until we signify the absence of cyclonic conditions. 

 It appears to be cloud which is the obstacle to a 

 pigeon's finding its way home, and possibly the same 

 conditions may be operative in natural migration. 



