PARENTAL CARE AND NEST-MAKING 289 



the activation of higher nerve-centres, which may imply 

 some degree of appreciative awareness. It is also uncertain 

 to what extent the bird may have acted intelligently or con- 

 trolledly in ancient days when a new departure in instinctive 

 routine was tested, and either approved of or rejected. 



The tendency to brood is a normal element in the cyclical 

 reproductive activities of the female bird, and is wrapped up, 

 in a way not clearly understood, with the activity of the 

 ovary. Normally, in the domestic fowl, the brooding 

 instinct finds expression after laying a clutch of eggs, but 

 it is not necessarily so. Raymond Pearl has shown (19 14) 

 that the degree of intensity of the brooding instinct, both in 

 respect of its objective manifestations and of its physiological 

 basis, may vary considerably at different times in the life 

 of the same individual. It is sometimes very persistent ; 

 in other cases it disappears very quickly. 



It has been shown that a pigeon will fail to notice its 

 eggs if removed to a distance of a foot from the nest, or that 

 another will sit close on a nest without any egg at all ; but 

 futility along particular lines is very characteristic of many 

 animals, and it means that the enregistration of certain chains 

 of behaviour may be so thoroughgoing that it approaches 

 mechanisation. When, in natural conditions, there is little 

 risk of anything going wrong with the routine, the more 

 automatic it becomes the more profitable it will be. 



It is characteristic of many instincts that they may 

 continue beyond the normal period unless some event occurs 

 which switches them off. This is well illustrated in the 

 case of incubation, and it is not always a disadvantage. In 

 their experiments with Tumbler Pigeons, Leon J. Cole and 

 W. F. Kirkpatrick noted (191 5) that when the eggs do not 

 hatch they are seldom abandoned at the end of the normal 

 period of incubation. The birds sit on for an average of 

 six days after the normal period, making the mean total time 

 of incubation (when the eggs do not hatch) twenty-three 

 days after the laying of the second egg. This is a " factor of 

 safety " in the incubating instinct. The number of days 

 required for the young to hatch being variable, " nature, 



u 



