222 Lewis, Some Considerations on Sight in Birds. [isf A^iii 



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 im- 

 mediately 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 one minute in degrees of the 

 circle, which means that at six metres we can distinguish clearly 

 enough to identify letters in lines one centimetre 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 about 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 

 becoming immobile, their protective coloaring blending with the 

 surroundings. 



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 \asual acuity can be seen in the Vulture and 

 its habits. On the death of an animal there may not be a Vulture 

 in sight, and in a 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 extremely 

 high fliers, only one bird out-soaring them — namely, the Adjutant. 

 It is probable that the nearest Vulture sights the animal, and 

 descends to the carcass. The bird's action is observed by the 

 Vulture further away, which is likewise led to 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 was 

 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 Brown Kingfisher {Dacelo gigas), from a position 

 on a post where it can inspect newly-ploughed land, seems to have 

 no difiiculty 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 Pigeons, feeding them on a board on wheat, among 

 which a percentage of -the grains have been stuck by adhesive 

 substance. One mistake is sufficient to prevent them again 

 making the error, small, slight alteration from the natural position 



