EYE MOVEMENTS IN BIRDS 307 



vergence, simultaneous. Like the turtles, the snakes prefer to scrutinize 

 objects binocularly, and even those whose binocular fields are narrow 

 will move the head from side to side in pendulum fashion, as if they 

 were trying the impossible of seeing all of the object with both eyes at 

 once. Dryophis and Thelotornis, probably Dryophiops as well, have 

 temporal foveae and enjoy foveal binocularity without benefit of con- 

 vergent eye movements (see pp. 185-6, 299). 



Birds, and the Visual Trident — For reasons which were pointed out 

 in Chapter 8, the bird eye is even larger than that of a lizard. It is a very 

 tight fit for its orbit, which could be called roomy only in the penguins 

 and cormorants. Only these birds, some other divers such as the pelicans 

 and gulls (and, strangely enough, the hornbills and ground hornbills) 

 have much eye mobility. Most birds have little or no spontaneous mobil- 

 ity, relying upon the flexibility of the neck; and even the reflex eye move- 

 ments may be greatly restricted and replaced by reflex neck movements. 

 In some cases, the eyes can turn reflexly in the vertical plane but not 

 when the head is rotated in the horizontal plane. Moving objects are 

 generally followed by movements of the whole head. Fixation may be 

 monocular with the central fovea, or binocular for optimal judgment of 

 distance — even in parrots, whose binocular field is very narrow. Such 

 spontaneous mobility as there may be is mostly horizontal, and for the 

 enlargement of the binocular field. Even the hen is capable of this slight 

 convergence, despite a 144° divergence of the optic axes. 



The imperative need for accurate distance-judgement, coupled with 

 the impossibility of any chameleon-like binocular use of the central 

 foveae, has led to specializations of the temporal part of the retina. 

 Some of these are slight, like the 'red field' of the hen; but in many 

 different groups of birds, independently of each other, a second fovea 

 has been differentiated in the temporal quadrant. It is present in the 

 very birds which, one might say from their feeding habits, need it most : 

 the various hawks and eagles, the humming-birds (Fig. 80b, p. 188), 

 the swallows, many bitterns, and various passerine wing-feeders. Despite 

 their close kinship with the hawks, the vultures apparently lack the 

 temporal fovea. Since they are ground feeders, this is readily compre- 

 hensible. The extra fovea of the kingfisher is believed to have a very 

 special significance (see p. 442). 



The accipitrine birds, the swallows, etc., thus have what Rochon- 

 Duvigneaud has called the Visual trident'. They look antero-laterally 



