2T4 THE MIGRATION OF BRITISH BIRDS 



lapped. Migration then would be limited, the birds 

 that visited the northern areas in the immediate neigh- 

 bourhood of the ice-sheets and snow-fields during the 

 short summer, would draw south in winter (just as the 

 Song Thrushes that now breed in Scotland move south 

 in winter), but the majority of the individuals would be 

 more or less sedentary. As the climate changed, and 

 a return to more genial conditions commenced, the 

 northern breeding range gradually expanded by emi- 

 gration, and the migrations as gradually became longer, 

 in many cases the breeding and wintering areas becom- 

 ing discontinuous (although continuous by Migration), 

 owing to change of climatic conditions, the North 

 becoming more suitable for successful reproduction 

 and summer residence, the South less so, but still 

 adapted for winter residence. 



All these facts tend to prove that Routes of ]\Iigration 

 are not only continuous, but that they were formed 

 whilst the species was extending its northern area by 

 emigration, and therefore represent the expansion of 

 breeding range which has taken place during Post- 

 Glacial time. The various Routes of Migration to the 

 British Islands may be divided into several very distinct 

 classes. First we have the Routes followed by Summer 

 Migrants — or of birds that come to our area in spring, 

 and depart from it in autumn. These are all situated 

 on our southern coasts. Then we have the North Sea 

 Routes, which arc chiefly traversed by the great stream 

 of east to west migrants that visit us in autumn 

 and leave us in spring; whilst, lastly, we have the 

 Routes followed by species that come to our area 

 to winter, or that pass over it on their wa}- to more 



