THE 'VISUAL TRIDENT' 



309 



owls from swift-like forms through the goatsucker and frogmouth types. 

 The situation in Apus (p. 188) suggests that the ancestors of the owls 

 may have had both foveae. A central fovea would be of little value to a 

 modem owl; for, owing to the great restriction of the visual angle in the 

 tubular eye, the angle between its line of sight and that of the temporal 

 fovea would be a narrow one. The owl eye cannot be turned in the orbit, 

 even with a pair of pliers. It has been pointed out that these, the most 

 frontal of all bird eyes, are the least mobile while the most frontal of all 

 mammalian eyes (our own) are the most mobile. This makes sense how- 



Fig. 115 — Projections of the visual fields of a hawk, showing the visual 

 trident of bifoveate birds. Relative resolving powers are suggested by the 

 closeness of the hatching, b- binocular field; m, m- residual monocular 

 (uniocular) fields; x, x- blind region; c, c- projections of the central 

 fovea; /- common projeaion of the tempxjral foveae. 



Fig. 116 — A 

 bittern, Ixobry- 

 chus minutus, 

 in 'freezing' 

 posture, show- 

 ing ability to 

 see binocularly 

 beneath the 

 head. Redrawn 

 from a photo 

 in LIFE. 



ever when it is kept in mind that the owl's head can swivel through 270 

 or more; and this situation actually does have its parallel among the pri- 

 mates — in Tarsius, whose tubular eyes are immobile and whose head can 

 rotate on the neck through an angle of 180°. 



The owl is safe enough in the matter of distance-estimation, without 

 having a visual trident — for it does have the all-important central tine of 

 the trident; and its almost bat-like ability to dodge obstacles, through the 

 use of auditory cues, enables it to avoid crashes as easily as does a hawk. 

 But the necessity of the temporal foveae (with or without the rest of the 



