CHAP. II. ] BIRDS. 21 
We see, then, that migration is governed by certain intelligible 
laws; and that it varies in many of its details, even in the same 
species, according to changed conditions. It may be looked 
upon as an exaggeration of a habit common to all locomotive 
animals, of moving about in search of food. This habit is greatly 
restricted in quadrupeds by their inability to cross the sea or 
even to pass through the highly-cultivated valleys of such 
countries as Europe; but the power of flight in birds enables 
them to cross every kind of country, and even moderate widths 
of sea; and as they mostly travel at night and high in the air, 
their movements are difficult to observe, and are supposed to be 
more mysterious than they perhaps are. In the tropics birds 
move about to different districts according as certain fruits 
become ripe, certain insects abundant, or as flooded tracts dry 
up. On the borders of the tropics and the temperate zone 
extends a belt of country of a more or less arid character, and 
liable to be parched at the summer solstice. In winter and 
early spring its northern margin is verdant, but it soon becomes 
burnt up, and most of its birds necessarily migrate to the more 
fertile regions to the north of them. They thus follow the spring 
or summer as it advances from the south towards the pole, feeding 
on the young flower buds, the abundance of juicy larve, and on 
the ripening fruits; and as soon as these become scarce they 
retrace their steps homewards to pass the winter. Others whose 
home is nearer the pole are driven south by cold, hunger, and 
darkness, to more hospitable climes, returning northward in the 
early summer. As a typical example of a migratory bird, let us 
take the nightingale. During the winter this bird inhabits 
almost all North Africa, Asia Minor, and the Jordan Valley. 
Early in April it passes into Europe by the three routes already 
mentioned, and spreads over France, Britain, Denmark, and the 
south of Sweden, which it reaches by the beginning of May. It 
does not enter Brittany, the Channel Islands, or the western part 
of England, never visiting Wales, except the extreme south of 
Glamorganshire, and rarely extending farther north than York- 
shire. It spreads over Central Europe, through Austria and 
Hungary to Southern Russia and the warmer parts of Siberia, 
