some of the hawks and many ducks and shore birds pay the valley scant 

 attention. They may arrest the autumn journey to feed among the 

 cottonwoods or along sand bars, but when ready to resume their flight 

 they leave the river and fly directly south over the more or less arid 

 region that lies between the Arkansas and the Rio Grande. 



Wide and narrow migration lanes 



When birds start their southward migration the movement neces- 

 sarily involves the full width of the breeding range. Later there is 

 a convergence of the lines of flight taken by individual birds, owing 

 to the conformation of the land mass, and as the species proceeds 

 southward the width of the occupied region becomes less and less. 

 An example of this is provided by the common kingbird, which breeds 

 from Newfoundland to British Columbia, a summer range 2,800 miles 

 wide. On migration, however, its paths converge until in the southern 

 part of the United States the occupied area extends from Florida to the 

 mouth of the Rio Grande, a distance of only 900 miles, and still farther 

 south the migration path is further restricted. In the latitude of Yuca- 

 tan it is not more than 400 miles wide, and it is probable that the great 

 bulk of the species moves in a belt that is less than half that width. 



A migration route, therefore, may be anything from a narrow path 

 that adheres closely to some definite geographical feature, such as a 

 river valley or a coast line, to a broad boulevard that leads in the de- 

 sired direction and which follows only the general trend of the land 

 mass. Also it is to be remembered that whatever main routes are 

 described, there remains a multitude of tributary and separate minor 

 routes. In fact, with the entire continent of North America crossed 

 by migratory birds, the different groups or species frequently follow 

 lines that may repeatedly intersect those taken by others of their own 

 kind or by other species. The arterial routes, therefore, must be con- 

 sidered merely as indicating paths of migration on which the tendency 

 to concentrate is particularly noticeable. 



In considering the width of migration lanes it will be obvious that 

 certain species, as the knot and the purple sandpiper, which are nor- 

 mally found only along the coasts, must have extremely narrow routes 

 of travel. They are limited on one side by the broad waters of the 

 ocean, and on the other by land and fresh water, both of which are 

 unsuited to furnish the food that is desired and necessary to the well- 

 being of these species. 



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