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 larvae, 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, 



