142 THE MIGRATIONS OF BIRDS 



tinctly eastern forms to the Mackenzie River Valley 

 and even into Alaska. 



In western North America we find, in general, a 

 wide, poorly defined flight line, which carries birds 

 through the Plains area or the Rocky Mountains 

 southward into Mexico. The northern portion of 

 the Great Basin, particularly the region about Great 

 Salt Lake in Utah, has proved to be an area of con- 

 centration in autumn migration, particularly for 

 migrant ducks and shore-birds coming from the 

 north. Such birds concentrate in these great 

 marshes, and then in subsequent flight may travel 

 southeast, south, or southwest. There appear to be 

 two main flight lines, one that crosses the mountains 

 to the western border of the Plains and then sweeps 

 southward, and another that passes to the west and 

 southwest through areas of desert, to cross the 

 Sierra Nevada and pour a vast flood of birds into 

 the trough of the interior valley of California east of 

 the Coast Range. The return flight in spring, while 

 abundant, is of somewhat different character, since 

 many birds that come south through this region 

 seem to migrate north by other lines. The early 

 spring flights have not been adequately studied, and 

 will no doubt with close observation yield unsus- 

 pected facts. It is possible that a number of northern 

 water-birds not supposed to pass commonly through 

 the interior use the Bear River marshes at the north- 



