All travel of mouse, man, or bird has three dimensions: length, or dis- 

 tance of travel ; height, the elevation of the eyes that guide travel ; breadth, 

 the distance between the horizons on either side of the line of travel. These 

 coordinates vary with every animal because of variations in structure; the 

 elevation of the eyes is half an inch in mouse, about five and a half feet in 

 man, and anywhere from a few inches to several thousand feet in the Can- 

 vasback. The breadth of each individual's travel is relative to its height.* 

 We are able to understand these values all the better if we contemplate a 

 ten-mile trip from the level of a mouse, then step into a light aircraft to 

 experience the same journey as a Canvasback. 



There is still another travel coordinate: the motion of the traveler. 

 Length, height, and breadth merely outline the space within which a jour- 

 ney takes place, just as the walls of a coop are the chickens' boundaries of 

 movement therein. Coops may be built according to ten thousand plans, 

 each with different dimensions. Travel likewise follows many patterns, but 

 travel implies movement within its three dimensions, and length, height, 



• Breadth of travel may be modified by ground haze or other meteorological factors limit- 

 ing visibility. Breadth is also variable according to variations in acuity. Breadth of vision may 

 be extended by mountain ranges; but in the absence of these, even under conditions of ideal 

 visibility, recognizable detail at the horizon approaches zero because of tangential viewing. 

 Thus, for these several reasons, the exact mathematical measurements of breadth of travel can- 

 not be given. This, however, does not greatly detract from the importance of this dimension at 

 elevations normally used by birds in migration. 



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