28 



and comers, where they passed the winter in sleep. 

 Another widespread belief was that large birds — 

 Storks, Herons, and Cranes — whose departure each 

 autumn was manifest to all, carried off their smaller 

 brethren on their backs or otherwise. 



Nowadays we have advanced a step beyond this 

 stage; but, as to the primar\^ as well as the more 

 immediate causes of the seasonal migration of birds, 

 our knowledge is sliil largely confined to speculation, 

 albeit speculation of a type somewhat different to that 

 of our ancestors. To these causes we sliall revert, 

 after we have considered some of the facts of which 

 we are cognizant. 



We know, then, that a large number of birds all 

 over the world change their abode according to the 

 season. As regards our own countr\% we find that 

 some — for example, the Nightingale — arrive from the 

 south in spring to breed, and depart in autumn ; 

 others, such as the Fieldfare, whose nesting areas 

 reach the Arctic Circle, spend the winter with us, and 

 return thither in spring. Others, again, commonly 

 called " birds of passage," appear but twice a year, in 

 autumn and in spring, passing through the country 

 without staying in it, on their way to and from their 

 breeding places farther north and their winter quarters 

 farther south. Many of these are reared amid arctic 

 snow, and yet annually visit the tropics : the Knot and 

 the Sanderling are examples. 



Even such sedentary birds as our Robin, are, on 

 careful investigation, found to be in some degree 

 " partial migrants," although that term is usually con- 

 fined to those species, a few individuals only of which 

 remain with us all the year round. Great flocks of 

 Robins arrive every autumn on our eastern coasts from 

 the Continent, and many also leave us at the same 

 period for more southerly countries. 



