20 STUDIES IN BIRD-MIGRATION 



Just in proportion as the ice retreated, would the birds 

 push forward the bounds of their habitat. He is careful 

 to tell us, however, that the first migrations of birds date 

 from an earlier period than the glacial epoch, for many 

 species were already birds of passage before its advent. 



Migration Routes. — It is quite reasonable for -the 

 uninitiated to suppose that when journeying between 

 these more or less widely separated seasonal homes, the 

 migrants move indiscriminately southwards in the 

 autumn, and, in like manner, northwards in the spring. 

 Such is not the case. Each individual migrant of 

 mature age has its accustomed summer and winter 

 quarters, and follows particular and more or less devious 

 routes to reach them. As the ways of many are identical, 

 great and much-used fly-lines are followed. As long ago 

 as 1846, the celebrated German ornithologist, J. F. 

 Naumann, expressed himself thus [Rhea, 1846, p. 18) on 

 this important subject: "There must," he says, "even 

 be highway roads through the air, which are annually 

 taken by migrants, certain spots on the earth beneath 

 these tracts serving the travellers as stations for rest 

 and recovery. In other tracts again migrants are 

 either strikingly scanty or altogether absent." These 

 "migration routes," as they are termed, have been 

 carefully studied for Europe and northern Asia by 

 Professor Palmen in particular, and are shown on the 

 accompanying map (Plate II.). It will be observed 

 that coast lines and the courses of great rivers are 

 followed, lofty mountain ranges and wide belts of desert 

 are traversed, and lesser or vaster expanses of sea are 

 crossed. Hosts of migrants, even delicate Warblers and 

 other birds of feeble wing, thread the snowy passes of 

 the Himalayas when seeking and retreating from the 



