W. H. THORPE 77 



that with further trials the bird's positive response to bait and or string 

 tended to increase over a period of time. Subsequently the bird's response 

 to both might tend to decrease and even birds which were learning well, 

 or in which the string-pulling habit had been completely mastered, might 

 be set back, or the habit lost, when the experimental situation was changed 

 or the bird was moved to a new cage. 



Further tests, not only with wild and hand-reared blue and great tits 

 but also with greenfmches {ChIori<; chloris), chaffmchcs [Friti'^iUa coelebs) 

 and canaries [Serimis c. canarius), have thrown a good deal of light on 

 what is evidently a very complex situation. It seems clear that juvenile 

 birds tend to be superior to adults in the string-pulling situation and that 

 this success can be correlated with the amount of time spent responding 

 to the string aiid/or the bait. This amount of time responding could be 

 due to a higher level of activity, and in tasks requiring a high level of 

 activity (requiring, in fact, large persistence and many trials) juveniles — 

 which are at their maximum of exploratory activity at about 13 weeks — 

 are likely to be more efficient than adults. But positive response is not the 

 whole of learning, and in such a task as string-pulling it is necessary also 

 for the bird to learn to refrain from responding to situations in which 

 reinforcement is absent or withdrawn. This ability to refrain is, as Vincc 

 shows, dependent on something similar to what Pavlov called internal 

 inhibition.^ It is weak in very young birds, develops as a result of age, and 

 as a result of experience during the juvenile stage, is still being strengthened 

 at an age when positive responsiveness is well past its peak and later 

 weakens sHghtly again. Consequently tasks which depend less on the level 

 of activity, and more on the ability to control behaviour by precisely 

 timed inhibition are more likely to be mastered quickly and efficiently by 

 older birds than by younger ones. Other experiments in which great tits 

 were required to obtain food by opening dishes covered with white lids 

 and refrain from opening dishes covered with black lids gave further 

 confirmation of this view. These studies also suggest that internal inhibi- 

 tion is unstable in young birds and that the process of development from 

 the unstable type of inhibition found in younger birds to the more stable 

 type found in older birds, depends not only upon age but also on experi- 

 ence. 



In another experiment, great tits were first trained to feed from a blue 

 dish. When they had become habituated to the blue dish, a white 



^ A full discussion of the relationship between Habituation and Internal Inhibition would 

 be out of place here; those interested will find it dealt with at some length in my book 

 (Thorpe, 1956, 57-62). 



