220 THE MIGRATIOX OF BRITISH BIRDS 



breeding area. It is significantly enough reported by 

 the keeper of the South Maidens Light, in the very 

 centre of this North Channel, off the Irish coast, that 

 "no birds strike the lantern in April and May." This 

 fact also implies that none of our summer migrants 

 reach Scotland by way of Ireland. 



The routes followed by these Summer Migrants very 

 plainly indicate the physical changes which have taken 

 place in the British Area, and also the approximate date 

 of the first arrival of such species as emigrants in Britain. 

 In the first place, we know that migrants continue to 

 cross wide water areas in such cases where the line of 

 Migration had become established before submergence 

 had made any appreciable alteration in the physical 

 character of such areas — hence the persistent passage of 

 birds across the North Sea from and to our islands. In 

 the second place, we have very ample proof that Emigra- 

 tion or the extension of breeding range is rarely if ever 

 attempted across wide water areas, such increase of 

 summer area having perhaps invariably been accom- 

 plished before submergence took place and such water 

 divisions came into existence. Keeping in mind these 

 two very important conditions, we are able to deduct 

 the following interesting facts from a study of the known 

 Routes of migratory birds in spring to our islands. 

 First, that the birds which cross the widest water areas 

 are the descendants of those species that were the first 

 to colonize or extend their breeding range to our area 

 after the Glacial Epoch. At the present day these are 

 few both in number of species and individuals, and they 

 are found still to continue to enter Ireland in the 

 extreme south-west, having reached that area when the 



