S42 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



visual axis of one eye, when a double image is obtained. In the 

 human it is possible to suppress the vision by exercise and education, 

 otherwise the eye must be closed — thus, in shooting or looking down 

 a microscope — but by a continual effort at concentration it is possible 

 to keep both eyes open and to suppress the vision of one. 



When we come to acuity of vision in birds one must immediately 

 recognize a superiority over the rest of the animal kingdom. There 

 is no doubt that they possess an acuity almost immeasurable compared 

 with our own standard. Normal sight in man gives an acuity of 

 about 1 minute in degrees of the circle, which means that at 6 meters 

 we can distinguish clearly enough to identify letters in lines 1 centi- 

 meter in width. Man and monkeys are perhaps in advance of the 

 rest of the mammals, but fall extremely short of the standard found 

 in birds. Speaking roughly, it is justifiable to say that birds possess 

 ubout a hundred times the degree of acuity found in man. Visual 

 acuity for moving objects is much more keen. This probably accounts 

 for the habit of small animals or birds wishing to escape detection 

 };ecoming immobile, their protective coloring blending with the sur- 

 I'oundings. 



Peep through the smallest hole in a fowl-yard fence, and one will 

 find that some old hen has perceived the action. An instance of the 

 remarkable visual acuity can be seen in the vulture and its habits. 

 On the death of an animal there may notbe a vulture in sight, and in 

 u few hours' time many will have arrived at the feast. These birds 

 become aware of a dead beast not by smell (as that sense is vestigial) , 

 but by sight. Vultures are extremel}^ high fliers, onh^^ one bird out- 

 Sioaring them — namely, the adjutant. It is probably that the nearest 

 vulture sights the animal and descends to the carcass. The bird's 

 Jtction is observed by the vulture farther away, which is likewise led 

 fo the scene, and so it goes on. In this way it is believed that birds 

 come from a distance of from 50 to 100 miles by their observation of 

 each other's action. A fact pointing to their ability to locate a carcass 

 w\as observed in one of the outbreaks of rinderpest in Natal. It was 

 found that if a carcass were covered by branches immediately after 

 death, so as to obscure it from the sight of the birds, it was never 

 disturbed by vultures. 



Though there is no means of measuring accurately the visual acuity 

 of birds, a fair idea may be obtained by observation of their habits. 

 A great brow^i kingfisher {Bacelo glgas)^ from a position on a post 

 where it can inspect newly plowed land, seems to have no difficulty 

 in locating the exposed part of a worm from any distance up to 100 

 yards. Watch an old hen in charge of a few chicks, and nothing 

 overhead, be it ever so small, will escape her notice. 



Acuity for stationary objects, though not so finely sensitive as for 

 those moving, is still remarkable. Experiments have been made with 



