71 



varying between such divergent points as north-east to south." 

 This, of course, does not affect the truth of Herr Gatke's statement 

 that migration is performed in greatest strength, or is most ap- 

 parent at the surface of the earth, during the prevalence of light 

 south-easterly winds accompanied by fine warm weather, but it 

 effectually disposes of his contention that these conditions form 

 the incentive to small numbers of certain eastern species to for- 

 sake their customary routes of migration in favour of a flight from 

 the far east to the far west. 



Mr. Eagle Clarke further explains that strong westerly winds 

 are unfavourable to migration simply because they are the result 

 of types of pressure-distribution which are fatal to migration 

 between North-western Europe and Britain, i.e., the conditions 

 prevalent in the former area, during their prevalence, would be so 

 disturbed that birds would be prevented from setting out. 



Herr Gatke, on p. 85, refers to the deterrent effect of fog on 

 migration at Heligoland. Judging from his remarks, fog either 

 prevents migrants from setting out at all, or induces them to 

 perform their flights at great elevations. This is not always the 

 case, however, for he mentions instances, both in his diaries and 

 in the present work, of birds in large numbers being heard 

 migrating above the fog. 



Of course it is well-known that local fogs are far more 

 frequent than fogs of great extent which envelop large tracts of 

 land or wide areas of sea. Whilst the latter may prevent birds 

 from starting on their flights, the former can only have a very 

 unimportant influence on migration in general. It is easy to see, 

 however, how a fog surrounding so small a spot as Heligoland, 

 even if its extent was limited to the immediate neighbourhood 

 of the island, might so hide the latter from the view of passing 

 birds as to prevent them making their usual call. 



With regard to the effect of fogs on the British coasts Mr. 

 Eagle Clarke remarks, that during their prevalence more birds 

 than usual approach the lanterns of lighthouses, and that many 

 are killed on such occasions. These facts point to the correctness 

 of the theory that migration, as a rule, is conducted at only 

 moderate elevations. 



Reverting again to the altitude of and general conditions affect- 



