xvi. 28 BEHAVIOUR 481 



is adequately proved, but whatever distinction exists is presumably a 

 reflection of the difference between the functions of large masses and 

 spread-out sheets of nervous tissue. Certainly birds can count as well 

 as rodents, and their performance in mazes and puzzle boxes is at 

 least comparable to that of most mammals. 



There are many signs that a suitable initial stimulus sets off in the 

 bird a whole train of behaviour, organized from within. On the other 

 hand, inappropriate stimuli may sometimes set off a reaction, as if the 

 'keys' to these cerebral 'locks' were not very elaborate or specific. A 



Fig. 291. Pintail ducks (Anas acuta). Males displaying dark brown feathers 

 of neck with white bands on either side. (From Tinbergen, after Lorenz.) 



robin held in the hand may burst into song, cock ostriches frightened 

 by an aeroplane fall to the ground in their characteristic sexual display. 

 Birds frightened or disturbed may proceed to the actions of bathing, 

 preening, feeding, or drinking, performed in a ritual and cursory 

 manner for a long time. Such displacement activities show that the 

 organization of the bird's nervous system, like that of a mammal, pro- 

 vides for some strange deviations, whose study may reveal much about 

 the method of working of the brain. 



Many complex forms of behaviour are responses to only limited 

 parts of the natural stimulus situation. Thus when the models shown 

 in Fig. 290 were towed above certain young birds, only the models 

 marked with a cross induced escape reactions: apparently the con- 

 figuration of the short neck is the essential feature. Much of the 

 elaborate social life of birds depends on such sign stimuli displayed 

 by one bird (the 'actor') and serving as releasers setting off particular 

 actions or trains of action in another bird (the 'reactor'). Many of the 

 elaborate forms of display evolved by birds (p. 497) are releasers of 

 this sort (Fig. 291), and structures and actions on the part of the 

 young release the appropriate behaviour of the parent. The red breast 



