78 BRAIN MECHANISMS AND LEARNING 



cardboard lid was placed over it. The number of trials (i.e. the reinforced or 

 training trials) needed for the consistent removal of the white lid was then 

 recorded. Unreinforced trials consisted of empty dishes with black lids. 

 When hand-reared birds were compared in this test with wild-reared ones 

 caught asjuveniles over a period of 8-20 weeks after fledging, both groups 

 showed internal inhibition (indicated by the ability to refrain from remov- 

 ing the black lid) rising as a function of age and later falling slightly. But 

 there are indications that a richer and more varied experience may 

 change the slope of the curve, giving a sharper rise to a higher level. It 

 seems as if, at any rate in this experimental situation, richness of early 

 experience is an important factor in contributing to the perfect mastery of 

 a task. Thus in a task primarily requiring superabundant activity (as, for 

 instance, does maze learning, where a large number of different cues have 

 to be investigated), younger animals arc more likely to excel. On the 

 other hand, a task which requires activity directed to a particular feature 

 of the environment may well be learned better by older animals. Both 

 these features arc shown in the studies of bird learning which I have been 

 describing. 



To sum up, there are four aspects of development which have emerged 

 from such experiments. Firstly, there is the question of responsiveness; 

 this appears, in the hand-reared great tit, to increase with age and then 

 decrease. Secondly, there are changes in a different type of responsiveness, 

 namely habituation or internal inhibition. Here again there appears to be a 

 rise and also probably a slight subsequent fall with age, but the rates of 

 the first and second changes may be quite different, and so the variation in 

 these two factors couJd well give rise to the puzzling differences in learning 

 ability which the earlier experiments revealed. Thirdly, there is the 

 question of the hunger drive. The birds in this work were kept under 

 conditions such that approximately the same measure of food deprivation 

 was experienced by all. Fourthly, the effects of early experience or 

 environment on behaviour are clearly shown and it seems that internal 

 inhibition may develop more rapidly and more completely in a more 

 varied environment, presumably as a result of greater activity, greater 

 stimulation and perhaps greater opportunity for adaptation and so on. It is 

 plausibly assumed that aviary rearing is a richer experience than hand- 

 rearing and that rearing in the wild is a richer experience still. 



hiipriiitiin^. The phenomenon of imprinting is now so well known to the 

 student of bird behaviour that a definition is hardly necessary. Recent 

 experiments have confirmed that the period during which young birds 

 can tirst learn to follow moving objects is strictly limited. But once a 



