VISUAL ANGLES AND FIELDS 295 



abundantly under water; and in Cerberus he was unable to detect any 

 power of accommodation. 



Among the birds we may distinguish straight-headed forms like the 

 pigeon and the song-birds, whose eyes are laterally aimed (Fig. 70, p. 

 172), from round-headed predaceous species such as swallows, goat- 

 suckers, hawks, and owls, with more or less frontality — the optic axes 

 never diverging more than 90° (Fig. 115, p. 309). Some penguins 

 (Spheniscus spp.) have no binocular field, and consequently weave and 

 sway a good deal when they are scrutinizing an object. Others, like the 

 Adelie penguin, look binocularly at far objects and when walking, also 

 at near objects when they are angry; but they turn the head sidewise and 

 look monocularly in any calm examination of a near object. Whether 

 the shoe-bills, toucans, and such birds have had to sacrifice all binoc- 

 ularity for the sake of their huge bills, is not known. 



The parrots have the smallest binocular fields of any so far measured 

 in birds — 6° to 10° in most species. An exception, of course, is the flight- 

 less, nocturnal owl parrot or kakapo of New Zealand (Strigops habrop- 

 tilus) which has strong frontality and a considerable (but unmeasured) 

 binocular field. Another New Zealand bird is quite unique: the rare 

 blue or mountain duck, Hymenolaimus malacorhynchus. Whereas all 

 other ducks fixate monocularly, this species has the eyes aimed forward, 

 and fixates binocularly like a hawk. 



Granivorous birds never have over 25 of binocularity, and many 

 have less than 10°. The homing pigeon, for instance, has been found 

 to have a 24 binocular field upon full convergence, with a total field of 

 340°- 342°. In line with the generalization stated above concerning pre- 

 dacity, the insectivorous birds and herons have higher values and in the 

 hawk group the binocular field varies from 35° to 50° or more. Owls 

 have 60°- 70° ; and considering their marked frontality the hawks and 

 owls would have even wider binocular angles were it not for the fact that 

 their monocular fields are so restricted by tubularity. The round-headed 

 ostriches and their allies also have wide binocular fields but no exact 

 figures are on record. 



The most exceptional birds are the snipes, as exemplified by the wood- 

 cock. Every hunter knows that in the bizarre 'timber doodle' the eyes are 

 set far back on the head — so far that the posterior binocular field is prob- 

 ably much wider than the anterior. The bird's feeding habits afford an 

 explanation: the long bill is thrust so deeply into the ground after worms 

 and the like, that the bird would be most vulnerable to attack when feed- 



