THE PERCEPTION OF FORM 663 



This superb acuity is not, of course, universal among birds. Thus, 

 testing the vision of domestic hens to see a grain of wheat in strange 

 surroundings, Engelmann (1952) concluded that the limiting value was 

 determined by a retinal image 0-02 mm. in diameter. Nor is their 

 form sense, despite the excellence of its physical basis, always fully 

 exploited. Conditioning experiments have been undertaken on a 

 considerable scale in birds, particularly the pigeon, a research pioneered 

 by Popov in Pavlov's school (see Razran, 1933) (ten Cate, 1923 ; 

 Beritoff, 1926 ; Riddle and Burns, 1931 ; To we, 1954 ; Jones, 1954 ; 

 and others). It has been established that birds are eminently trainable 

 to distinguish between different kinds of geometrical figures of equal 

 area, and that the development of their sense of form is relatively high. 

 At the same time, when pigeons are offered a choice of a number of 

 visual variables in discriminative problems they always respond con- 

 sistently to one of the variables only. Jones (1954) established that 

 cues based on colour were most readily followed, those depending on 

 position came next, while form discrimination was the most difficult 

 to learn. 



The excellence of the form perception of birds is also seen in their 

 extraordinary powers of recognition. This is a well-attested phenome- 

 non ; birds rapidly learn to recognize each other even when two weeks 

 old (the coot, Fulica, Alley and Boyd, 1950) and recognition is often 

 made entirely on a visual basis even when the bird in question is 

 silent. Robins {Erithacus) can recognize their silent mates at a distance 

 of over 30 yards even although they are partially screened by trees 

 (Lack, 1939) ; tits {Parus) can distinguish individuals in a flock at 60 

 yards distance (Morley, 1942), while pintails {Dafila) can identify one 

 another 300 yards away (Hochbaum, 1944). An artificial change of 

 appearance as by transferring the comb to the side of the head, may 

 destroy recognition (see Thorpe, 1956). Recognition of human beings 

 by birds is also well kno^ii, the facial characteristics sometimes being 

 recognized in spite of a change of clothing (Poulsen, 1944 ; Buxton, 

 1946 ; Ash, 1952 ; Thorpe, 1956). In this respect also the visual 

 memory may be long ; it is true that in some species impressions may 

 fade after a few days, but jackdaws can remember individuals for 

 several months (Lorenz, 1935), a pigeon has been said to remember a 

 particular person after 11 months (Diebschlag, 1940), and a hen trained 

 to eat off a certain colour performed her task again a year after the 

 training had ceased (Claparede, 1926). The annual return of many 

 migratory birds to the same spot is another case in point. 



Most MAMiNiALS are in an entirely different category ; only the 

 Sciuridse (the entire retina of which may be said to be a macula), 

 a few Carnivores, some Ungulates, and Primates have a highly developed 

 visual acuity. Thus in rats and mice training experiments show that 



