ENVIRONMENT, PAST AND PRESENT 49 



Non-breeding birds may migrate, but typically 

 theirs is but a partial migration. Since several 

 species are known to breed in immature plumage 

 and to complete their migrations it may be difficult 

 to assess correctly the status of immature migrants. 



Many writers include feeding movements, if of 

 sufficiently impressive range, in the term migration. 

 Such movements may vary from the few yards 

 covered by the limpet as it creeps from its perma- 

 nent seat to the nearest patch of seaweed and back, 

 to the hundreds of miles traversed by many shoals 

 of fish as they accompany and prey on swarms of 

 plankton drifting on some ocean current. Animals 

 in general must move to obtain their food ; move- 

 ment may, in fact, be considered an integral part of 

 the business of feeding. Whatever the distance 

 covered, the principle is the same and if such move- 

 ments are to be admitted into the category of migra- 

 tion any attempt at definition must ultimately 

 break down It is not always a simple matter to rec- 

 ognise purely feeding movements and this applies 

 particularly in the tropics. In severe winters in 

 the British Isles extensive movements may take 

 place, but they are carried out under stress of im- 

 mediate circumstances, during winter, not before 

 it, are quite sporadic and frequently entail a heavy 

 mortality in all of which they are in contrast to 

 true migrations. 



