followed the Asiatic coast, through the Malay Peninsula and Oceania, 

 thence east in a great curve to the Low Archipelago, with individuals 

 and flocks dropping out to winter at many points along the way. The 

 Siberian birds probably continue to follow this ancient highway, but 

 those nesting in Alaska began a long evolutionary series of flights that 

 cut down the length of their journey by shortening the curve, until 

 finally the transoceanic route of the present day was developed. 



This theory of the evolution of migration routes has been questioned 

 by some ornithologists on the ground that it implies the possession in 

 some degree of reasoning powers such as would be used by human 

 beings. This opposition suggests that changes in migration routes 

 might develop suddenly following mass survival of birds that were 

 driven over the new route by a storm on some specific occasion. In 

 the language of genetics, the new route would be, in effect, a mutation, 

 lather than the result of an accumulation of infinitesimal variations. 

 There is some evidence in support of this opposing theory. For exam- 

 ple, information from the Hudson's Bay Co. post at Great Whale 

 River, on the southeastern coast of Hudson Bay, indicates that in 1884 

 the snow geese suddenly changed their route from the eastern to the 

 western coasts of Hudson and James Bays. According to one report, 

 this change was caused by strong winds from the south which caught 

 the birds in their fall migration and caused them to cross the entrance 

 of James Bay from Cape Jones to the western side; the route thus re- 

 portedly forced upon them was then used in succeeding years. 



Vertical Migration 



In the effort to find winter quarters furnishing satisfactory living 

 conditions, many North American birds fly hundreds of miles across 

 land and sea. Others, however, are able to attain their objective merely 

 by moving down the sides of a mountain. In such cases a few hundred 

 feet of altitude correspond to hundreds of miles of latitude. Move- 

 ments of this kind, known as "vertical migrations," are found wherever 

 there are large mountain ranges. In the Rocky Mountain region they 

 are particularly notable, as chickadees, rosy finches, juncos, pine gros- 

 beaks, and some other species that nest in the Alpine Zone move down 

 to the lower levels to spend the winter. It has been noted that such 

 species as Williamson's sapsucker and the western wood pewee, which 



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