gained much in the lateral position of the eyes. At high altitudes depth 

 perception is little needed. Close to the earth, however, and particularly 

 when alighting, there must be some awareness of the three-dimensional 

 character of the environment. Pumphrey (1948:182) has pointed out that 

 such depth perception hinges on time and motion, that "the appreciation 

 of distance has to be built up by a succession of glances from different 

 points toward the same point of the field." He refers, of course, to parallax. 

 The moment the bird is in flight, the position of the eye in relation to objects 

 in the landscape is ever changing. The most rapid changes are in the nearest 

 objects, hence the relativity of motion is the clue to the depth of the land- 

 scape. "Near objects seem to move extensively in the opposite direction as 

 compared with far objects, while the latter seem to move slightly, in the 

 same sense as the head movements, in relation to the nearer objects. It is 

 chiefly this cue which enables a one-eyed man to move about in an un- 

 familiar roomful of furniture without bumping into things any more often 

 than a two-eyed person" (Walls, 1942:314). 



We human beings employ parallax daily in measuring our environment, 

 and it must be the most important clue the bird has in the perception of the 

 landscape's three-dimensional character. For birds the motion of travel 

 establishes the parallax; the more rapid the flight, the more keen is the 



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