ROUTES OF MIGRATION. 89 



past experience has taught it to be the best for its 

 own requirements, or the countless contingencies 

 and necessities of migration have compelled it to 

 learn. Consequently, we come across many apparent 

 anomalies in studying the fly-lines of birds ; we 

 shall find them intricately interlaced and even 

 crossing each other at right angles; running 

 parallel but reversed by certain species according to 

 season ; turning without any apparent cause. If 

 it were only possible to focus the Palaearctic or 

 Nearctic region into a bird's-eye view during the 

 months that migration is in progress in spring and 

 autumn, and to be able to see the various species 

 en route, each following its chosen course, the whole 

 movement would appear to be in the wildest 

 confusion. Birds would be seen flying this way 

 and that, crossing each other's path, or following it 

 for a certain distance, then quitting it again, passing 

 each other in opposite streams, or gathering in 

 certain localities to separate again as soon as they 

 were crossed, flying from many points of the com- 

 pass, intricately mixed, yet all orderly following the 

 chosen path which leads to the desired destination ! 

 As we have already seen. Migration is not a 

 capricious habit ; we shall soon learn that the 

 Routes which are followed are almost as ancient as 

 that habit, and are adhered to in a very persistent 

 way. Broadly speaking, the majority of migratory 

 birds visit northern regions in spring and southern 

 regions in autumn (so far as the Northern Hemi- 

 sphere species are concerned) ; there is avast double 



