ISLAXD AVIFAUNAS 195 



Grouse was retained in winter owing to the pluvial 

 climate of the mild western region — a change which 

 the bird has been quick to take advantage of as a 

 means of protecting itself from enemies amongst its 

 native heath and ling. The propensity of the Red 

 Grouse to perch occasionally in trees is very interesting, 

 and unquestionably a relic of the arboreal habits of its 

 ancestors. The emigrations of the Red and Willow 

 Grouse, it will be remarked, are almost exactly analogous 

 to those of the Eastern and Western Nightingales. 



Of the remaining four endemic birds, the St. Kilda 

 Wren (if it can be preserved from extermination) has 

 probably the best opportunity of asserting finally its 

 specific distinctness from the Wren that inhabits the rest 

 of the British Area. It is, however, wrong to presume 

 that St. Kilda owes its Wren to a party of birds driven 

 south from Norway in search of a milder winter climate. 

 In the first place no species, nor single individual of a 

 species, normally migrates south along an unknown 

 route which its Post-Glacial emigrations north have not 

 followed. No bird, no species, emigrates or extends 

 its area in winter, colonizing movements are prompted 

 solely by reproduction.^ Of course it may be urged 



1 One most convincing proof that species do not increase their 

 area during winter, or retreat by emigration from adverse conditions, 

 is furnished by the fact that great numbers of species, or portions 

 of species, resort to winter quarters much further south or more 

 remote from their breeding grounds than there is any necessity so 

 to do. To my mind this is proof beyond the possibihty of doubt 

 that these remote winter areas are ancient range-bases from which 

 the species has been dispersed, and from which it has colonized the 

 most remote parts of its present area. The Willow Wren, for 

 instance, goes in some numbers as far south as The Cape to winter, 

 but many individuals remain in North Africa and even in South 



