90 THE MIGRATION OF BIRDS. 



exodus from north to south, and from south to 

 north, every year. Any person ignorant of the 

 subject might infer, and reasonably so, that this vast 

 army of pilgrim birds passed to and fro without 

 any regard for order or route ; that each strove as 

 best it could to reach the summer home or the 

 winter quarters. But the very reverse is the case. 

 For birds follow certain routes, and in some cases 

 even modify their habits considerably to do so ; 

 some inland species becoming for the time being 

 littoral ; others, marine species, quitting their coastal 

 habitat to follow a certain fly-line. This persistent 

 attachment to old routes of migration is exemplified 

 in a very remarkable manner by the present 

 geographical distribution of certain species, as I 

 hope presently to show. 



The great routes of migration whether over land 

 or sea are closely connected with the configurations 

 of the earth's surface. We may for the sake of 

 convenience divide them into four very marked 

 classes, viz. : Sea Routes, Coast Routes, Mountain 

 Routes, and River or Valley Routes. The first of 

 these highways is the one followed more especially 

 by aquatic birds ; land birds are not known in any 

 part of the world habitually to cross seas for much 

 more than 400 miles, unless there are intermediate 

 stages in the form of islands on which they may 

 rest, if necessary. The two longest routes of 

 migration over the sea which land birds are known 

 to follow, are first in the Atlantic Ocean between 

 the Azores and Portugal, a distance of 900 miles ; 



