WHAT DO ANIMALS SEE? I47 



image on to the outer part of the retina, rich in rods, we are 

 able to see it at once. Most birds are in a worse case than 

 we are, for their retinas consist almost entirely of cones and 

 as a result they are nearly blind at night. For this reason 

 most birds go to roost before darkness has completely fallen, 

 and if disturbed during the night they blunder about 

 clumsily. Poachers have been known to take advantage of 

 the night-blindness of game birds. 



In daylight, however, most birds enjoy an acuity of vision 

 much better than ours — they can see small objects four or 

 five times further away than we can. The cones at the fovea are 

 very numerous and closely packed, and the pupil of the eye 

 is relatively large. The large size of the pupil tends to de- 

 crease the sharpness of the vision because it emphasizes the 

 colour fringes at the outlines of objects, but this defect is 

 corrected by the presence of coloured oil droplets in the cones 

 which filter out the unwanted fringes without abolishing 

 colour vision. The central fovea in birds lies in a compara- 

 tively deep depression in the retina, and it has been suggested 

 that the sides of this pit act as a lens to magnify the size of 

 the part of the image falling upon it. This is probably true 

 to a limited extent, but the advantage of the deep fovea is 

 more likely to be in emphasizing the movement of the image 

 of an object crossing the line of sight so that "fixation" is 

 enhanced. This makes it easier both to pick up a moving 

 object and to avoid losing it when once picked up, especially 

 against a featureless background such as the sky. 



Fishes and reptiles also have a deep central fovea and they 

 too are quick to respond to movement, but do not react to 

 a static field of vision. It is well known that snakes generally 

 will not eat dead prey which lacks the movement of the 

 living, but they can be induced to do so in captivity by 

 waggling the offered food before them. One may speculate 

 too whether the to-and-fro movement of the head that is 

 characteristic of many birds when they walk may move the 

 visual picture across the retina and so bring different parts 

 of it in succession on to the fovea. This might help grain- 

 eating birds to find the seeds and other small objects on 

 which they feed. 



Birds that chase moving prey in the air have eyes that are 



