156 THE MIGRATION OF BIRDS. 



various berry-eating species, such as the Fieldfare 

 {Turdiis pilaris) and the Missel-Thrush (Turdtis 

 viscivorus). All these birds undertake journeys of 

 varying length during winter in quest of food — 

 movements not exactly of a migratory nature, yet 

 sufficiently regular and important to require notice 

 in connection with the usual migration flight of 

 birds. Again, birds of far-extending and regular 

 migration wander about their winter quarters after 

 their long Flight is done, visiting this district and 

 that according to the abundance of food. 



It is very difficult to classify these Local Move- 

 ments, or to determine which are regular migrations 

 and which are not. It would seem that a certain 

 amount of local migration is actually in progress 

 even during mid-winter, as the evidence gathered by 

 the Migration Committee of the British Association, 

 by Gatke on Heligoland, and other observers, is 

 absolutely undeniable. Nor is it confined to the 

 colder regions of the world, for instances are not 

 wanting of Winter Flight in the tropic zones. It is 

 evident that a considerable amount of Winter Flight 

 takes place over the North Sea, to and fro between 

 the Continent and the British Islands, especially 

 among Waders and aquatic birds ; and this move- 

 ment is probably due entirely to failing food supply, 

 or severe weather in either district respectively. 

 A long spell of severe weather sends great flights 

 of birds from one district to another where milder 

 conditions prevail. I have repeatedly observed 

 instances of this winter migration during severe 



