44 THE MIGRATION OF BIRDS 



from spreading to districts apparently well suited 

 for its home, and until recently the turtle dove and 

 great crested grebe were checked in their northward 

 advance. 



In the evolution of some routes land-bridges 

 certainly appear to have played their part, but once 

 those bridges have ceased to influence direction the 

 shortening of the time occupied by the lengthening 

 of the single oversea flight is only a question of 

 generations when an advantage to a species is to be 

 gained. 



This subject will be further dealt with in connec- 

 tion with the actual passages performed by certain 

 birds. 



The study of migration, based on observations at 

 our lighthouses and lightships, shows that even in 

 the comparatively small area of the British Islands 

 there are certain routes followed with regularity. 

 The birds which pass along our western coasts of 

 England and Wales do not as a rule follow the shores 

 round Cardigan Bay or along the eastward tidal 

 scoop of the Irish Sea towards the coasts of Lanca- 

 shire ; the main body passes from Pembroke to the 

 Lleyn Peninsula, and thence to Anglesey and the 

 Isle of Man, on its way to the southern Scottish 

 shores. 



A source of possible error in the method of deduc- 

 tion from these results must be taken into con- 



