I30 THE MIGRATION OF BIRDS. 



variety of reasons, as we have already seen, birds 

 are apt to dispose of surplus population by irruptic 

 emigration. Flocks of such birds we have every 

 reason to believe occasionally wander long distances 

 from their usual habitat, ready to settle in any 

 favourable locality they may chance to discover. 

 That such wandering birds sometimes cross wide 

 expanses of sea is also certain ; and it is to these 

 nomads that the presence of birds on certain islands 

 lying out of the track of normal migrants is almost 

 if not entirely due. These birds settle in their new 

 home; and owing to isolation from the rest of 

 the species, any variations that may arise, excited 

 primarily by changed conditions of life, are pre- 

 served through the absence of interbreeding, and 

 in the process of time become constant characters. 

 Thus we find in many islands endemic species of 

 birds obviously descended from parent forms in 

 adjoining but isolated areas, and to which they 

 are more or less closely allied. 



Islands located far from routes of migration are 

 almost entirely populated, so far as birds are con- 

 cerned, by fortuitous emigration. There is no 

 regular influx of individuals, as is constantly taking 

 place at the two seasons of passage on islands 

 situated on or near a great route of migration, as 

 for instance at the Bermudas, or even the British 

 Islands, with the inevitable result of keeping the 

 sedentary portion of the avifauna true by inter- 

 breeding. Hence we almost invariably find that 

 the great proportion of the species are endemic, yet 



