48 A NATURALIST ON DESERT ISLANDS. 



mere effects of isolation, ^dll, together with other things, 

 lead in the end to changes of plumage and dimensions in 

 these birds, if they remain permanently resident, just 

 as they have done in the case of the Canary Islands chiff- 

 chaff (P. rufus fortunatus), a sub-species of our European 

 chiff-chaff. 



But how came these birds thus to drop their migratory 

 habit ? Did the chmate and the conditions generally, 

 in the Canaries, gradually come to fulfil exactly throughout 

 the whole year the requirements of the chiff-chaff, and so 

 gradually do away with the necessity for periodical 

 migration ? Under these conditions we can conceive 

 that those birds which did return annually to Europe 

 in the spring would gradually become fewer and fewer, 

 until at length there would be none left, and this migratory 

 branch-route north to south or south to north would 

 cease to exist, and the Canary Island birds would be cut 

 off from any autumnal influx of birds which had bred 

 in the north, and would be completely isolated. 



But if this is the explanation in the case of the Canary 

 Island chiff-chaff, it can hardly hold in the case of the 

 myrtle-warbler and redstart flycatcher, which must surely 

 have found the conditions obtaining in summer, on the 

 Caymans and Dominica respectively, totally different 

 from the conditions they had been accustomed to on the 

 continent during their breeding season. 



Why, again, pick and choose in such a capricious way ? 



In what manner, for instance, do the conditions found in 



Cuba differ from those of the Caymans, or those found in 



the islands immediately to the north and south of 



Dominica, from those of Dominica itself ? Yet in Cuba 



we find no variety of the prairie-warbler which is resident ; 



and Dominica alone, out of all the Antillean Islands, 



appears to have been chosen by Setophaga ruticilla for 



a permanent residence. 



******* 



This reference to the subject of migration leads us on 

 to consider the migratory visitors to Swan Island. Every 



