ROUTES OF MIGRATIOX. 103 



birds come from considerable distances inland, 

 following local routes or bye-ways to join the great 

 trunk road which all then follow in common 

 according to their specific time. 



On the other hand, however, equally large 

 numbers of species, and even of individuals, whose 

 habitat is in the interior of mighty continents, are 

 too far removed from such coastal highways, either 

 to be aware of their very existence, or to render a 

 cross flight impracticable, and these birds have 

 naturally chosen other configurations of the earth's 

 surface to serve as great trunk roads for their 

 annual migrations. Unquestionably the most im- 

 portant of these are the great river valleys w^hich 

 sere the earth's surface from north to south in so 

 many countries visited by migratory birds. To a 

 very great extent all the small tributaries which 

 drain the haunts of these migrants are follow^ed 

 until the main valley is reached and the trunk 

 fly-line joined. On all the great continents there 

 are river valleys known to be crowded with migrants 

 passing along them in spring and autumn. In 

 Europe the most important River Routes are as 

 follow^s. Firstly, the valleys of the Petchora, north 

 and south Dvina, and Onega, which lead to the 

 various upper waters of the Volga, tlie Don, and 

 the Dnieper, along which all the migratory land 

 birds of Russia journey south to the Black and 

 Caspian Basins, and thence (by the coast routes 

 of those great inland seas) to East Persia, Asia 

 Minor, and Egypt. Secondly, the Vistula, the 



