INTERNAL MIGRATIONS. 157 



weather. During long-continued snowstorms all 

 our Sky Larks have vanished ; frosts of long 

 duration invariably banish the Redwing (^riirrlus 

 iliacus), the Snipes {Scoiopax), and other ground- 

 feeding species; failure of the berry supply will 

 initiate a local migration of all birds that chiefly 

 depend on it for subsistence. On the other hand, 

 during severe weather flocks of other birds have 

 visited us that seldom or never do so under ordi- 

 nary circumstances. We shall enter more fully into 

 this in a later chapter. 



From the above series of facts we may learn that 

 few birds are really stationary throughout the year; 

 that it is rather the exception for a species to be 

 absolutely sedentary. Again, many if not all young 

 birds are great wanderers, driven from their birth- 

 place by their parents, or deserting it voluntarily as 

 soon as parental care becomes unnecessary. One 

 important result of all this Local Movement is, that it 

 serves to keep individuals well mixed together, and 

 insures that all-necessary cross-breeding, or "mixed 

 marriage," which is so essential to the well-being 

 and even preservation of each and every species. 



