248 THE MIGRATION OF BIRDS. 



towards the coast of Belgium, on the old route back 

 to the East. Another very important migration, 

 chiefly of these early migrants, is also, incredible as 

 it may appear, actually coming into England from 

 the Continent at the same period ; so that we have 

 the astounding phenomenon of a marked and 

 constant Migration across the North Sea in exactly 

 opposite directions ! These latter migrants are birds 

 that evidently breed in our islands, but for some 

 reason prefer to winter on the Continent. This 

 movement may be due to the influence of Temper- 

 ature varying on individuals of the same species. 

 The Crows, Starlings, Larks, Buntings and Finches 

 spending their summer in Great Britain, may require 

 a higher winter temperature, and retire towards 

 Southern Europe to obtain it ; wdiilst those from 

 the colder east and north area of continental land, 

 and naturally of more robust constitution, find the 

 temperature suited to their individual requirements 

 in our islands. In autumn this Cross Migration 

 may in part be due to the stream of migrants from 

 North-western Europe, which passes along our 

 coasts to the south, meeting the east to west stream 

 of migration, composed of birds on their way to 

 winter with us. Whatever the dominating cause, 

 the fact is unquestionable, and the Cross Migration 

 is not only regular but persistent, especially during 

 seasons remarkable for great and sudden changes 

 of temperature. I firmly believe that temperature, 

 <7^ affecting individuals, is the chief initiating cause 

 of these singular Avian waves, and that their ebbing 



