662 



THE EYE IN EVOLUTION 



Pigeon 



Shrike, Lanius 



The highest visual acuity in the entire vertebrate phyhim is seen 

 in BIRDS ; this we would expect with their enormously large eyes with 

 an anteriorly placed lens and a globular posterior segment, their 

 emmetropic refractive condition and magnificent accommodative 

 mechanism, the multiplicity of oil-droplets in the cones, the excellence 

 of their foveae, the perfection of the lamination and the low summation 

 of their retinae. This is indeed the case, for the visual resolution 

 attained by some of the passerine wing-feeders and the predators is 

 phenomenal. Investigating this problem, Pumphrey (1948) estimated 

 that a resolution of about 10" of arc should be possible by the avian 

 retina, three times the accuracy attainable in the human retina, and 

 in training experiments, Grundlach (1933) actually demonstrated a 

 resolution down to 23" in pigeons ; in these birds a high degree of 

 form -discrimination can be developed although it tends to be primarily 

 unidimensional (Chard, 1939 ; Towe, 1954 ; Jones, 1954). In this 

 connection it is to be remembered that the degree of resolution capable 

 by a bird such as the hawk ought to be of a considerably higher standard 

 than that of the pigeon. 



This potentiality is borne out in the everyday activities of birds 

 (von Hess, 1912; van Eck, 1939; Rochon-Duvigneaud, 1943 ; Donner, 

 1951 ; and others). It is true that many insect-catchers such as the 

 swallow or the night-hawk trawl for their food indiscriminately on the 

 wing particularly during the twilight hours with little reliance on vision; 

 but the visual acuity of the martlet which flies high and at intervals 

 swoops downward upon an individual insect at a considerably lower 

 level, or that of the humming-bird which opens its long narrow beak 

 but slightly and impales minute insects individually with its long bifid 

 tongue, must be superb. In many birds the visual acuity far exceeds 

 that of man ; the reactions of fear by the shrike, Lanius, which the 

 falconer carried with him in a cage, let him know the whereabouts of 

 his bird of prey long after he himself had lost track of it in the sky. 

 Even an owl, the eye of which is specialized for night vision,^ will 

 detect a hawk approaching in the day-sky at a height at which it is 

 invisible to man. The excellence of the optical resolution of which the 

 avian eye is capable is probably aided by a markedly high capacity 

 to differentiate tones, a faculty possibly based on the light -filtering 

 effect of the oil-droplets of their cones ^ ; thus dead game lying on the 

 ground, to us completely camouflaged by its surroundings, will be seen 

 by the African vulture — and it will recognize that it is dead — from a 

 height of 3,000-4,000 metres, a height so great that a man cannot 

 discern the bird in the sky with its 3-metre wing-span. 



^ See p. 605. It is to be noted that according to v. Hess (1912) the retina of the owl 

 contains 2,500,000 cones. 

 2 p. 631. 



