134 THE MIGRATIOX OF BRITISH BIRDS 



the Eastern migrants being later in their arriv^al. Were 

 it only possible to separate the individuals, I think we 

 should find that so far as the typical Eastern migrants 

 are concerned, they all journey practicall}' about the 

 same time, and come from the same areas. I think this 

 phenomenon of an East to West migration in autumn 

 will also explain why such species as the Crane {Grns 

 connnunis), the Bittern {Botaiinis stellaris), and the 

 Great Bustard {Otis tarda), are now only winter visitors 

 to our islands. All these species a century or so ago 

 were common summer visitors to the British Islands, 

 the individuals breeding in our area migrating south in 

 autumn to their accustomed winter quarters ; but on 

 the other hand, our islands were visited in winter by other 

 individuals of these three species that were returning to 

 their old quarters, from whence their ancestors emigrated 

 East in past ages, and which they continue even to the 

 present day to regard as their winter home. Several 

 other instances confirming the above remarks might 

 also be given. 



Singularly enough, the individuals breeding in the 

 British Islands. of man}- of the species (perhaps of all) 

 tabulated above migrate south in autumn, their places 

 being taken b}* these migrant individuals of the same 

 species from the east. This seems to be a strangely 

 anomalous fact, yet it is one that shows how very com- 

 plicated the whole subject of Avian Migration undoubt- 

 edly is. As I said in the I\Iigration of Birds (p. 248), I 

 am strongly disposed to think that Temperature has a 

 good deal to do with this very complex movement. 

 Besides, there can be no doubt that the individuals of 

 these species that breed in Britain are later arrivals in 



