438 BRITISH BIRDS. 



tops of lofty trees, and is undoubtedly a more difficult bird to shoot. Like 

 the Siberian Chiffchaff, it hurries through the woods as if its object were to 

 cover as much ground as possible. Its flight is not so rapid as that of the 

 Willow- Wren, but is more undulating, the rapid motion of its rounded 

 wings apparently requiring frequent short rests. 



Its food consists of gnats, small beetles, and caterpillars, and of small 

 insects of all kinds, which it generally picks up on trees, but sometimes 

 takes in the air or on the ground. It seems to be more confined to woods 

 and plantations than the Willow- Wren, only venturing into large gardens, 

 and seldom visiting the stunted trees on the edges of the moors. In 

 autumn, when the young are fledged, it will come into the gardens to feed 

 on the currants, or frequent the underwoood in the plantations to regale 

 itself with elder-berries. 



The special interest attaching to the Chiffchaff is that it is one of the 

 earliest summer migrants to land on our shores, and in the cultivated 

 districts, where the Wheatear is seldom seen, is the first bird of passage to 

 announce to us the return of spring. 



It seems at first sight difficult to imagine how two such closely allied 

 birds as the Willow-Wren and Chiffchaff, which differ so little in their 

 geographical distribution, could have become differentiated. But if we 

 assume that the common ancestors of the two species lived in Europe 

 before the glacial period, we may conjecture that when the ice drove them 

 across the Mediterranean, half of them took refuge in the valley of the 

 Nile, whilst the other half were isolated in Algeria and the surrounding 

 countries, which then probably formed a large island. During the hundred 

 and fifty thousand years that this state of things is supposed to have con- 

 tinued, the colony in Algeria may have had time enough to develop into 

 Chiffchaffs, whilst that in the valley of the Nile became Willow- Wrens. 

 A third colony may have been isolated in Turkestan, from which the 

 Siberian Chiffchaffs may be descended. Special circumstances in the 

 valley of the Nile may have caused the intermediate colony to alter more 

 than the two outside ones, which may resemble each other because they 

 both have changed but slightly from the common ancestors. After the 

 glacial period was over, each colony would naturally follow the retreating 

 ice, and again spread over its original area of distribution, the central 

 colony overlapping in its new area that of its eastern and western rivals, 

 and possibly destined eventually to supersede and exterminate them. The 

 eastern range of the Chiffchaff reaches the western limit of the Siberian 

 Chiffchaff, about longitude 50; but the Willow- Wren covers the area of 

 distribution of both birds in the breeding-season, except perhaps the 

 Canary Islands in the west and the basin of Lake Baikal in the east. 



In the south of England the Chiffchaff arrives about the end of March, 

 in Yorkshire early in April, and in Edinburgh (according to Macgillivray) 



