360 THE STRUCTURE AND LIFE OF BIRDS chap. 



Acrocephalus turdoides, our Great Reedwarbler, is found in 

 summer as far north as the south of Sweden. Its winter mi- 

 gration takes it to the Transvaal and even further south. 



A. orientalis differs little except in having a slightly less 

 pointed wing. It migrates from Japan to Borneo. 



A. stentorius. Wing much more rounded. It migrates only 

 from Turkestan to India. 



A. syrinx has the roundest wing of all. It does not migrate, 

 but is resident in the Island of Ponape. 



It is highly probable that among individuals of the 

 same species similar differences exist, that those 

 which travel furthest on migration are better fitted 

 by the shape of their wings for long voyages than 

 those which have a less extended range. This has 

 actually been observed, it is said, in the case of the 

 Wheatear and the Willow Wren. It is very natural 

 that such variation should be found if there is truth in 

 the theory that among the birds of a particular species 

 those that winter furthest south pass the summer 

 furthest north. Mr. Seebohm gives strong evidence 

 of this. The Swallows at Natal start for the north 

 the last week of March, only those that were hatched 

 the previous spring setting out later, in the first half 

 of April. But the swallow returns to southern 

 Europe by the end of January, and in Spain Mr. 

 Howard Saunders found many broods hatched by 

 April 1 6th. It seems clear, then, that the Natal 

 Swallows do not stop till they have made their way 

 further north, and that our own, which arrive in the 

 first half of April, when those which stop in Spain 

 are well on with their nesting, have come from the 

 far south. A priori, too, it is probable that birds 

 which spend their summer in the cool northern climes 



