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 airor 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 cou- 
tinned, the colony in Aigeria 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 
elacial 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) 
