the coast of Mackenzie, and finally 700 miles south — in spring — to the 

 region near the eastern end of Great Slave Lake. 



The yellow-billed loon is a powerful flier, and it is probable that 

 this suggested route is correct for those birds that breed in the northern 

 coastal regions. A reasonable doubt may be entertained, however, 

 whether the breeding birds of Great Slave Lake and contiguous areas 

 reach their breeding grounds by the 700-mile flight south from the 

 Arctic coast. Within recent years it has been found that these birds 

 are fairly common in the maze of channels and islands ofT the coast of 

 southeastern Alaska as late as the last of October and in February. 

 Possibly they are present there during the period from November 

 through January also, or they may at that time move farther off shore 

 and so escape detection. If this region is an important wintering 

 ground, as seems probable, then it is likely that the breeding birds of 

 the interior reach their nesting grounds by a flight eastward across 

 the mountains, a trip that is well within their flying ability, rather than 

 by a circuitous route around the northern coast. The air-line distance 

 from southeastern Alaska to the mouth of the Liard River is in fact 

 less than the distance to that point from the mouth of the Mackenzie. 



Differing routes to various parts of a large breeding or wintering 

 ground, and used by large groups of individuals of other species, are 

 not unknown. For example, the redhead duck is one of the common 

 breeding ducks of the Bear River marshes of Utah, where a great many 

 have been banded each summer. The recovery records of banded red- 

 heads show that while many travel westward to California, others 

 start their fall migration in the opposite direction and, flying eastward 

 across the Rocky Mountains, either turn southeast across the plains 

 to the Gulf of Mexico, or deliberately proceed in a northeasterly direc- 

 tion to join the flocks of this species moving toward the Atlantic coast 

 from the prairie regions of southern Canada. 



Conclusions 



The migration of birds as it is known today had its beginning in 

 times so remote that its origins have been entirely obscured, and it 

 can be interpreted now only in terms of present conditions. The 

 causes underlying migration are exceedingly complex. The mystery 

 that formerly cloaked the periodic travels of birds, however, has been 

 largely dispelled through the fairly complete information that is now 



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