io8 THE MIGRATION OF BIRDS. 



mighty torrents which testify each recurring season 

 to their vital importance as Highways of pilgrim 

 birds. The river iiy-lines equal in length those 

 of the longest coast routes ; and although in our 

 present state of knowledge we cannot trace the 

 absolute route taken by more than a few individual 

 species, we are enabled from our general inform- 

 ation respecting their geographical distribution 

 broadly to determine the general course of a very 

 great number of others. River valleys are ex- 

 ceptionally favourable migration routes. The great 

 variety of species following them can obtain abund- 

 ance of food either on their waters, on their banks, 

 or amidst the rich vegetation which clothes the 

 slopes above the stream. Thus, we find that birds 

 of all kinds follow the course of rivers ; land birds 

 and water birds, the insect- or seed-eating Passeres, 

 the swamp-loving Waders, the aquatic Ducks, 

 even the oceanic Gull or Tern — all are equally 

 favoured. They also enable birds to reach their 

 breeding-grounds in the Arctic regions at the 

 earliest possible moment, which is of the greatest 

 importance in a land where summer, if hot and 

 brilliant, is remarkably short. 



It remains for us now to notice Mountain 

 Routes. Although these are perhaps followed least 

 frequently of all, we have a considerable amount of 

 evidence to prove that they are not only widely 

 used, but of very great importance. As we have 

 already seen, mountain chains in many cases act as 

 landmarks and guides to the migrating birds that 



