480 THE BIRDS xvi. 28 



cortex and top of the striatum are said to prevent incubation (though 

 not copulation) and with deeper lesions the whole process of rearing 

 the young becomes impossible. 



These observations on the functions of the brain of pigeons may not 

 be applicable to birds in general. The forebrain is larger in many other 

 birds than in the pigeon, and there is some evidence that in the parrots 

 movements and even 'phonation' can be elicited by electrical stimula- 

 tion of the corpora striata, which are especially large. Removal of one 

 particular area is said to lead to disturbances of 'speech'. 



B 



B 



Fig. 290. A. Models used by Lorenz and Tinbergen. Small birds reacted with 



escape movements to the models marked + . B. The model induced escape reactions 



when towed to the right ('hawk') but not when towed to left ('goose'). 



(From Tinbergen.) 



It seems, therefore, that the large masses of nerve-cells in the striata 

 are concerned in some way with the elaboration of the more complex 

 acts of behaviour. This is a very vague statement, but is the best that 

 we can give at present. It may be that further investigation of the 

 reciprocal actions of striatum and thalamus will show whether the 

 essentials of their action consist in some reverberating or scanning 

 systems and whether these actions are at all similar to those in the 

 forebrain of mammals. The fact that the striatum consists of solid 

 masses of tissue suggests that the arrangement does not depend, as 

 does the mammalian cerebral cortex, on the projection of patterns of 

 excitation onto an extended surface. 



Birds are usually said to show more stereotyped patterns of instinc- 

 tive behaviour than primitive mammals. Once they have embarked on 

 a line of action, even a complex one like nest-building> they are sup- 

 posed to pursue it in a given manner, without ability 10 adapt them- 

 selves to unusual happenings. Watching the exploratory behaviour of 

 a robin or a tit it is difficult to feel that the existence of this difference 



