MIGRATIONS OF WATERFOWL 



and ground speed may remain constant, but the ground flow is much slower 

 at 2,000 feet. It is by this apparent motion of the earth that both man and 

 bird perceive their progress, and the height of the eye is thus of great im- 

 portance in travel orientation. 



A man would find it impossible to direct his journey at 4 m. p. h. if his 

 eyes were at mouse level. The more rapid the ground flow, the more difficult 

 the problems of orientation, a fact we human beings acknowledge when we 

 slow down (or gain altitude in an airplane) when traveling in strange sur- 

 roundings. To walk at 4 m.p.h., at an eye level of 5& feet, gives a man a keen 

 and efficient awareness of motion through space. When a Canvasback flies 

 at 60 m.p.h. at an elevation of 8232 feet, its apparent ground flow is pre- 

 cisely the same as for the walking man (Figure 21). In short, 4 m.p.h. at 

 53a feet is to man as 60 m.p.h. at 823£ feet is to Canvasback; the 300 miles to 

 Lake Christina in 5 hours at an elevation of 8232 feet are to a Canvasback as 

 20 miles in 5 hours at 532 feet are to a man, the relative values of distance, 

 time-in-space, and height being the same for man and bird. The Canvas- 

 back seldom flies so low as this, almost never in migration, and it is immedi- 

 ately apparent that its ability to gain altitude gives it a tremendous advan- 

 tage over man. There is no need to resort to mathematical comparisons to 

 show how the bird is favored at 2,000 feet, but we must understand that not 

 only is the area seen from this altitude greater, but, because of the relative 



Figure 21. Relative travel equality for man and bird. At elevations and 

 speeds shown above the relative velocity and the apparent ground flow 

 are exactly the same for man and bird. The bird's relative velocity be- 

 comes less as it gains altitude, and it may rise so high that the apparent 

 ground flow becomes almost imperceptible. 



152 



