TRAVELS OF WATERFOWL 



muscular adjustment of the lens. In birds both the lens and the cornea are 

 altered in accommodation ; hence two parts of the eye function to maintain 

 instant sharpness of both near and far parts of the visual field, and objects 

 rapidly approaching the eye are held in focus. 



The best experimental work, Pumphrey says ( 1948 : 185 ) , seems to estab- 

 lish that "there is very little difference between the color vision of birds and 

 man," but he goes on to explain why he believes that even though "the 

 limits of color vision seem to be the same for man and birds, it is likely, 

 nevertheless, that color sense plays a more emphatic part in the visual 

 sensations of diurnal birds than of man." Kalmbach (1943), who studied 

 the feeding responses of wild Mallards to colored grains, found that "water- 

 fowl have shown a degree of aversion to unnaturally colored grain, though 

 the discrimination by ducks is less emphatic than with passerine and galli- 

 naceous birds. This, it is felt, is due largely to the 'puddling' method of 

 feeding which operates probably as much by the sense of touch as by sight." 



Vanderplank (1934) and Wojtusiak (1949), among others, have pre- 

 sented what they believe to be evidence of avian awareness of infrared. 

 Griffin (1952a: 369) has made a careful review of the many studies of 

 visual sensitivity to infrared radiation, concluding that there is "strong evi- 

 dence that the visible spectrum of birds is approximately the same as that 

 of the human eye" and that "theories of bird navigation based upon sensi- 

 tivity to thermal radiation from bodies of low temperature scarcely deserve 

 serious consideration." Pumphrey (1948) concurs in this view. Supporters 

 of the view that birds are sensitive to infrared claim that such perception 

 accounts for the bird's ability to fly in heavy darkness or through fog; but I 

 have observed that waterfowl refuse to travel in dense fog or complete 

 darkness. 



The pecten, a fanlike structure in the posterior chamber of the eyeball, 

 is universally present in birds (Figure 4). Its function is still speculative, 

 and many theories have been advanced in regard to its role in vision. An 

 important lead was presented by Menner (1938), who observed not only 

 that the foliations of the pecten cast shadows on a functional part of the 

 retina but that these foliations, hence the shadows, varied with different 

 kinds of birds. The pecten proved to be the most highly developed in diur- 

 nal birds with good visual acuity, such as hawks, and the least in nocturnal 

 species. Walls (1942) remarked that before Menner, some thirty theories 

 had been advanced regarding the function of the pecten, and Griffin 

 (1953:227) offers a conjecture that might be given, he says, "the serial 



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