22 THE MIGRATION OF BIRDS 



increasingly severe winters, to return and attempt 

 to nest in the locality which had become unsuitable 

 for nesting. The spread of glaciation would be 

 gradual and so would be the annihilation of the 

 species, but the end would be sure. 



Birds which are cited as species which have 

 shown this remarkable attachment to home, have 

 disappeared before adverse circumstances the 

 great auk and the Labrador duck. 



From what little we do know about the behaviour 

 of our summer birds in their winter home, we may 

 safely conclude that their habits are similar to 

 those of winter visitors to Britain. Only in a few 

 species are there two restricted areas, two abiding 

 places or homes. The necessity of retaining a 

 secure home for the young and the care of these 

 young during their more helpless age keeps the 

 individual birds within a certain area during the 

 breeding season, but at all other times the bird is 

 more or less of a wanderer. The variation, how- 

 ever, of the wanderings is remarkable. For instance 

 the flocks of fieldfares, redwings, and some of the 

 finches which come to winter in the British Islands 

 wander continually from feeding ground to feeding 

 ground, remaining in one place only so long as the 

 food supply is plentiful. When there is a plentiful 

 harvest of beech-mast, chaffinches and bramblings 

 will linger near one clump or avenue of beeches for 



