THE VISUAL WORLD 



more nor less in their lives than the drifting leaves of a nearby poplar. 

 Shortly afterward the ducks suddenly stood up on the sand bar, some of 

 them swimming away. Looking westward I saw a bird of rapid wing-beat 

 approaching at low elevation. It was several seconds before I was sure 

 that this was a hawk and some moments more before I could be certain it 

 was a Duck Hawk and not another Pigeon Hawk. When the predator was 

 within two hundred yards, all the ducks were in the water. None flew and, 

 as the falcon passed over, most of them dived, some frantically in water too 

 shallow to cover them. After the hawk had gone on, a few of the Mallards 

 flew away, but most returned to their island to pick up the lazy thread of 

 their afternoon siesta. At this point I will not raise the argument whether 

 the enemy-fear was instinctive or learned. The matter of importance is that 

 these birds recognized their enemy long before I did. 



The photoreceptive cells of the eye lie within the outer layer of cells of 

 the retina and are of two types, the rods and the cones, so named because 

 of their shapes. The rod cells are more sensitive to light intensity, respond- 

 ing to much lower degrees of illumination than the cones. Within their re- 

 stricted range, however, "the cones are responsive to much smaller changes 

 in intensity; and it is the cones which are responsible for color vision. The 

 cone system is consequently suited for the registration of detail in a brightly 

 illuminated visual field. The rod system, because of its extensive summation, 

 is comparatively incapable of resolving detail but can register an achroma- 

 tic picture of the gross features of the field at illuminations so low that the 

 cone system is quite blind. We might expect, therefore, to find, as in fact 

 we do, that cones are predominant in the retinae of eyes which are exclu- 

 sively diurnal, and rods in eyes used only at night. Acuity (the power of 

 resolving detail) and sensitivity are inverse requirements which cannot be 

 fully satisfied in the same eye" (Pumphrey, 1948:177). By and large, the 

 waterfowl come as close as any bird toward meeting the mixed require- 

 ments of day and night vision. Their acuity is sharp and they are active 

 by day; and yet in many respects they are crepuscular birds, moving swiftly 

 and surely in the dusk of morning and evening. Ducks are active under a 

 full moon through the night, especially during the breeding season. But 

 on dark and moonless nights, with the added curtain of cloud, their travel 

 about the marsh comes to a halt. 



In birds, especially those swift of wing that feed in the open, it is most 

 important that there be a clear image of near and far objects at every in- 

 stant. In man the adjustment of focus is accomplished by an involuntary 



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