212 THE MIGRATION OF BRITISH BIRDS 



conditions along the line of their past emigrations or 

 extension of range ; in other words, the range shrank 

 back upon itself. Birds, be it remarked, do not extend 

 their range or emigrate to inej-ease tJie area of their 

 winter qiiarte7's. Paradoxical as it may seem, it is 

 nevertheless a fact, that birds emigrate and migrate 

 SOLELY TO breed ! Extension of range is prompted 

 exclusively by increase of breeding population, and 

 therefore takes place in spring, north in the Northern 

 Hemisphere, south in the Southern Hemisphere. Hence 

 we find birds much more sparingly dispersed in summer 

 at their breeding grounds than in winter at their winter 

 quarters, where they are often gregarious and exceed- 

 ingly crowded or abundant in small areas, due especially 

 to the influx of the young. Whether their winter area 

 is large or small depends entirely upon the localness or 

 otherwise of those conditions which were favourable to 

 breeding occupation prevailing during the time their 

 northern range was curtailed by glacial climates, or as 

 the breeding range spread across the world. The Turn- 

 stone, for instance, as an example of past widespread 

 southern breeding areas, and therefore present extensive 

 winter range, is a circumpolar species breeding chiefly 

 on the shores cf the Arctic Ocean ; yet in winter its 

 migrations extend ov^er all the coasts of the Southern 

 Hemisphere, which it inhabited as a breeding species 

 in emigrating north, and which there is much cx^idence 

 to suggest it still continues occasionall}' to use for repro- 

 ductive purposes, for it is said to have bred on Lord 

 Howe's Island, Robbcn Island off the south coast of 

 Africa, on the coast of South-west Texas, on islands in 

 the Red Sen, on the Balearic Islands, the Canaries,. 



