THE VISUAL WORLD 

 number '32'." He points out that if, as Kramer's studies (1952) suggest, the 

 sun is useful to a bird in navigation, then the pecten's function might be 

 to this end, just as a sextant serves man in his navigation. 



"What would be more useful to a bird than a built-in yardstick laid out 

 upon its retina, a series of fixed points against which the bird might learn to 

 estimate the angular distance of the sun above the horizon? Furthermore, 

 to guide itself by means of the sun a bird obviously must spend some time 

 looking at its intensely bright disk, a process which is uncomfortable, to 

 say the least, for the human eye. Hence perhaps the evolution of the pecten 

 with its plicated outline of densely pigmented tissue and with the rich 

 blood supply which would serve to carry away excess heat from the sun's 

 image if this image should fall on any part of the pecten. Considering the 

 extent to which the avian eye is fixed in its socket, in contrast to the great 

 freedom of rotation of the human eyeball, it is a plausible supposition that 

 the definite outline of the two pectens, plus the fixed points provided by 

 the foveas, might provide the bird with a visual frame of reference against 

 which the position and movements of the sun could be gauged." 



In the lives of all waterfowl the perception of movement is a very im- 

 portant function of the eye. Every hunter knows of the Mallard's immediate* 

 reaction to the slightest motion. He may tip his face or move his hand ever 

 so slightly; but even though the approaching bird may still be well beyond 

 gunshot, it alters its course or "flares" upward to avoid danger. Indeed, 

 one of the most important things in wildfowling is the art of remaining 

 motionless. The downy Canvasback shows a keen awareness of movement; 

 only a few days after being hatched it is able to strike swiftly to snap a fly 

 from the air. Waterfowl live in a world that is important to them in terms 

 of movement. At rest they must be ever aware of the motion of enemies, 

 companions, and living food. On the water, subject to the pressure of wind 

 and waves, they must perceive movement to maintain their local orienta- 

 tion. In the air it is the apparent flow of the world beneath that gives them 

 their awareness of movement through space. 



The eye of the bird is specially adapted to the perception of movement. 

 The panoramic vision of these waterfowl serves to introduce movement 

 to the eye over a wide span of the landscape. Their high visual acuity and 

 the breadth of the sharp retinal image likewise serve to give them a keen 

 awareness of motion. Movement is perceived in the changing illumination 

 of the visual field. If the background is moving ( cattails in a breeze ) , if the 

 color of the shifting object is quite the same as the background ( a brown- 



37 



