The geographical distribution of animals, with a study of the relations ...

chap, ii.] BIRDS. 21We see, then, that migration is governed by certain intelligiblelaws ; and that it varies in many of its details, even in the samespecies, according to changed conditions. It may be lookedupon as an exaggeration of a habit common to all locomotiveanimals, of moving about in search of food. This habit is greatlyrestricted in quadrupeds by their inability to cross the sea oreven to pass through the highly-cultivated valleys of suchcountries as Europe ; but the power of flight in birds enablesthem to crossevery kind of country, and even moderate widthsof sea ; and as they mostly travel at night and high in the air,their movements are difficult to observe, and are supposed to bemore mysterious than they perhaps are. In the tropics birdsmove about to different districts according as certain fruitsbecome ripe, certain insects abundant, or as flooded tracts dryup. On the borders of the tropics and the temperate zoneextends a belt of country of a more or less arid character, andliable to be parched at the summer solstice. In winter andearly spring its northern margin is verdant, but it soon becomesburnt up, and most of its birds necessarily migrate to the morefertile regions to the north of them.They thus follow the springor summer as it advances from the south towards the pole, feedingon the young flower buds,the abundance of juicy larvae, and onthe ripening fruits ; and as soon as these become scarce theyretrace their steps homewards to pass the winter. Others whosehome is nearer the pole are driven south by cold, hunger, anddarkness, to more hospitable climes, returning northward in theearly summer. As a typical example of a migratory bird, let ustake the nightingale. During the winter this bird inhabitsalmost all North Africa, Asia Minor, and the Jordan Valley.Early in April it passes into Europe by the three routes alreadymentioned,and spreads over France, Britain, Denmark, and thesouth of Sweden, which it reaches by the beginning of May.does not enter Brittany, the Channel Islands, or the western partof England, never visitingWales, except the extreme south ofGlamorganshire, and rarely extending farther north than Yorkshire.It spreads over Central Europe, through Austria andHungary to Southern Russia and the warmer parts of Siberia,It