xvi. 29 COLOUR DISCRIMINATION 487 



the object (Figs. 297 and 298). Foveas with steep sides are found in 

 birds of prey, kingfishers, and others that have very high powers of 

 detecting movement; a similar but less pronounced arrangement is 

 found in fishes and reptiles. It is probable that the primitive functions 

 of the eyes were fixation and detection of movement, rather than reso- 

 lution of detail and recognition of patterns. Some birds have one con- 

 vexiclivate and one flatter fovea (Figs. 295 and 296), the latter being on 

 the temporal surface of the retina and used in binocular vision. The 

 fovea is also flat in the retinae of primates with binocular vision; evi- 

 dently the optical errors of a curved fovea cannot be tolerated where 

 there is fusion of the two retinal images. 



Birds undoubtedly discriminate colours, apparently on a trichro- 

 matic basis similar to that of mammals. No other animals, except 

 perhaps primates, show such responsiveness to colour in their 

 surroundings, including the food and other members of the species. 

 In animals that move so freely recognition and attraction of the sexes 

 is more efficiently performed in this way than by touch or odour. 

 The cones of birds often contain red and yellow droplets, which may 

 heighten visual acuity by reducing the effects of chromatic aberration. 

 The droplets in the central area are always yellow. The presence of 

 droplets of various colours in adjacent cones may also increase powers 

 of discrimination. Sometimes the droplets are so arranged as to allow 

 accentuation of different contrasts in the parts of the visual field. The 

 lower part of the pigeon's retina contains red, the upper yellow filters, 

 increasing the contrast of blues and greens respectively, as required 

 for vision against the sky in the one case and the ground in the other. 

 There have been many investigations of the distribution of sensitivity 

 in the retina; probably many birds are rather insensitive to the blue 

 end of the spectrum. There is no truth in the suggestion that the eyes 

 are sensitive to infra-red radiation. 



Although the eyes of some birds are directed forwards, so that their 

 fields overlap, they are said not to have binocular vision and decussa- 

 tion of the optic tracts is complete. Perception of distance, a very 

 important function for the bird, must be performed in some other 

 manner. In many birds the eyes are directed sideways, and the fields 

 of view may even overlap behind the head, for example in waders. 

 This may serve to give warning of predators. Many functions have 

 been suggested for the most enigmatic organ of the bird's eye, the 

 pecten, a pleated highly vascular fold, projecting from the retina into 

 the vitreous. It is possible that the irregular shadow cast by this organ 

 provides, as it were, numerous small blind spots and hence by a 



