THE VISUAL WORLD 



directly and selects grain precisely with its bill. Often, when food floats on the 

 water, it reaches out with its bill neatly to take a morsel from the surface. 

 In sexual fights and in the frequent bodily encounters of courting birds, it 

 thrusts its bill straightforward in a well-aimed blow. Such direct actions sug- 

 gest depth perception for a narrow zone directly facing the duck. 



But it is obvious, both from the structure and the behavior of the duck, 

 that there is monocular fixation for all parts of the landscape beyond reach 

 of the bill. While writing this paragraph, I visited the winter pens where 

 there are more than a hundred ducks and geese of a dozen species. As I 

 entered the room, each turned its bill to one side or the other to view me 

 with one eye. Not a single bird had its forehead in my direction. So, at othei 

 times, the object of interest is examined with one eye. This is especiall) 

 noticeable when a hawk or an aircraft crosses over the pen and the birds 

 tilt their heads to watch. Often I have seen waterfowl tip their heads when 

 in flight, as when a band of Whistling Swans comes directly over my hiding 

 place, suddenly taking notice of my presence. A most interesting observation 

 of the monocular response of a pet parakeet to its visual world was related 

 to me by Robert L. Lillestrand. The bird, a pet of Roy P. Van Devener, 

 was released in a bedroom which had rose-colored wallpaper patterned in 

 a floral design with thin vines. The little parakeet, then only six weeks old 

 and with no experience of the outdoor world, attempted to alight on the 

 painted vines. The same thing occurred on later releases in the room, and it 

 was not till the bird was nine months old that it learned the impossibility 

 of using the painted stems as its perch. 



For a man there is little stereoscopic effect in the landscape beyond fifty 

 or sixty yards from the eyes. As an aerial traveler, the bird has lost little and 



Figure 3. Drake Ruddy Duck 



showing the eye position 



typical of waterfowl 



