196 THE MIGRATION OF BRITISH BIRDS 



that the movement to St. Kilda was perchance a 

 purely fortuitous one, but the possibility of that being 

 the case is destroyed at once by the fact that St. 

 Kilda was completely glaciated during the Ice Age, 

 and every bird exterminated from its surface. The 

 Wren reached St. Kilda by the line of Emigration 

 which we have seen extended from the British Isles to 

 Greenland, and by which same route the Faroe Isles 

 and Iceland received their Troglodytes borcalis. That 

 both these insular Wrens preserve their subspccific 

 identity, and (if the present conditions continue; may 

 ultimately attain complete specific rank, is entirely due 

 to the fact that Migration along this route — so far as 

 these subspecies are concerned — has ceased ; the Wren 

 (in its various subspecific forms) is now sedentary, and 

 isolation is therefore complete. The three subspecies 

 of Titmice that inhabit the British Area, however, may 

 very probably never rank higher than they are to-day, 

 because the migration of the continental (and parent }) 

 species continues to pass our area. So long as this 

 migration and consequent intermixture of individuals 

 continue, the swamping effects of intercrossing will 

 prevent complete segregation ; were the migration, 

 however, to lapse, isolation would doubtless soon enable 



Europe at that season. Numbers of Chiffchaffs go as far south as 

 Abyssinia to winter, whilst others spend that season even in the 

 South of France, Iberia, and Italy. Many Sedge Warblers winter 

 in Algeria and various parts of North Africa, others journey 

 thousands of miles further to the south in that continent. It is 

 nonsense to suggest that these birds are seeking a suitable winter 

 climate so far to the south, as many individuals of the same species 

 find suitable winter conditions at a distance measured by thousands 

 of miles nearer to their breeding grounds. Scores of similar 

 instances might be given. 



