RE.TIN* 



Figure 4. Eye of Mallard (twice natural 

 size), cut away to show position of 

 retina, lens, and pecten 



CORNLA 



the optic axis. . . . For man, the distance element enters into every glance, 

 but the appreciation of detail of the field has to be built up by a succession 

 of glances from the same point and directed to different parts of the field. 

 For the bird, detail enters immediately into every glance." Let us measure 

 the focal angle of our own human eye by looking up from this page to the 

 landscape beyond the window. To read the entire scene we must shift our 

 eyes back and forth. When the eyes are at rest, only the narrow field directly 

 in the line of vision is in focus. On either side of this narrow focal angle we 

 are aware of form, color, and movement, but these are out of focus and 

 may not be understood completely until the eye is moved. From its eye 

 structure, we must believe that the duck or goose viewing this same land- 

 scape perceives much more of it in sharp focus. In a single glance, then, the 

 duck obtains a sharply defined image of much of the landscape meeting 

 the eye. It perceives clearly in an instant what a man can understand only 

 after a series of shifting glances. Of course, we must remember that the 

 scanning movements of the human eye are very rapid, the speed compen- 

 sating in part for the narrow field of sharp vision. 



Man has long acknowledged avian superiority in visual acuity (sharp- 

 ness of vision), and the example of the falconer's experience is traditional. 

 "It was the habit of the medieval falconer to carry a caged shrike on his 

 saddle, to keep track of the falcon. As long as the shrike acted fearful and 

 excited, the hawker knew that his proud tercel was in sight — though not to 

 him!" (Walls, 1942:169). 



All who have lived close to waterfowl have witnessed their awareness 

 of objects we human beings could not perceive. One afternoon I sat watch- 

 ing a band of Mallards loafing on a lakeshore sand bar. In the distant sky 

 there appeared a hawk which, as it approached more closely, I identified 

 as a Pigeon Hawk. From the time this bird was first seen until, after pass- 

 ing within a hundred yards of the ducks, it disappeared in the east, I ob- 

 served no reaction on the part of the waterfowl. The hawk seemed nothing 



34 



