THE VISUAL WORLD 



awareness of depth. In some species there are instinctive body actions which 

 apparently function to increase the effect of parallax. Walls (1942:342) re- 

 minds us that some "birds when walking (fowls, pigeons, doves) and 

 others when swimming ( coots and gallinules ) make perpetual f orward-and- 

 backward oscillatory movements of the head. It has been claimed that the 

 eyes never actually move backward through space — the forward move- 

 ment of the body just cancels the backward movements of the head. Thus, 

 although the body moves forward steadily, the head moves forward through 

 space by jerks and pauses. In effect, the eyes obtain a rapid succession of 

 previews of the surroundings from constantly new angles. The forward 

 movements of the head being so quick, each new parallactic observation 

 of the field is made almost simultaneously with the preceding one, and the 

 exaggeration of the apparent relative motions of objects at different dis- 

 tances furnishes a basis for the estimation of distance and relief." 



While ducks and geese do not jerk the head as does the Coot, still their 

 heads are often in motion when traveling through dense cover. The head of 

 a swan is seldom idle as it swims. In low-flying ducks there often is an up 

 and down swinging of the head, and on several occasions I have seen a 

 Green-winged Teal skimming along just above the reeds with its head in 

 constant motion. J. C. Lynch, pilot-biologist for the U.S. Fish and Wildlife 

 Service, and a specialist in low flying, told me that it is most helpful to the 

 pilot of an aircraft at low elevations to look at the ground from first one 

 side of the plane, then from the other. This is termed "scanning" and is 

 especially useful in judging distance when bringing a light plane in for a 

 landing. 



Other clues to distance possibly are learned. The size of the retinal 

 image depends on the distance of the object from the eye, but as a guide 

 to distance the bird must have a familiarity with the object seen. Perspec- 

 tive is another clue which depends to some extent on a familiarity with the 

 components of the landscape. Aerial perspective, the variation in color due 

 to atmospheric influences, probably is used by birds in the perception of 

 distance. All these clues no doubt serve in establishing the awareness of 

 altitude when in flight. 



The structure of the eye has a profound influence on perception (Figure 

 4). Pumphrey (1948:174, 182) points out that the retina of the bird "is 

 shaped so as to lie almost wholly in the image plane, so that all distant ob- 

 jects within the visual angle are sharply focused on the photo-sensitive 

 cells, whereas in the human eye this is only true of objects lying close to 



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