70 THE MIGRATION OF BRITISH BIRDS 



southerly as the winters ahnost imperceptibly waxed 

 longer and colder, and exterminated all the northern 

 portion of the sedentary species; slowly the northern 

 breeding range of species after species became more 

 and more contracted as the food supply decreased, or 

 the summer temperature lowered. And so matters 

 went on until only our three Refuge Areas contained 

 representatives of the species that had been extermin- 

 ated, or whose northern range had been contracted. 

 For the most part these three areas were probably 

 inhabited by a sedentary avifauna ; but even at the 

 climax of this cold period I think there can be little 

 doubt that a certain percentage of the temperate species 

 in Refuge Area II. — the hardiest^ — -undertook a migra- 

 tion from south to north with each recurring summer. 

 With a great number of these hardy species INIigration 

 finally ended in Emigration, as the climate ultimately 

 became sufficiently genial for them to winter in safety. 

 There may also have been a considerable local migra- 

 tion within each respective area, north in spring, south 

 in autumn, gradually extended as the glacial conditions 

 passed away ; for to my mind it is difficult to believe 

 that birds did not respond in a migratory wa)- from the 

 south to the changes of the seasons with their accom- 

 panying advantages or perils. 



Post-Glacial Migration therefore must have originated 

 south of our area, and gradually extended north as the 

 range of species expanded in obedience to more genial 

 conditions, and in many cases have lapsed as the climate 

 moderated. 



Now with regard to the birds that reached Refuge 



