102 BEHAVIOR OF PIGEONS. 



young which have been weaned, may make advances to adults, or they may react 

 to bill-contact stimuli (finger-tips) which are normally totally inadequate. The 

 range of potential stimuli is wide, and inside of this range the stimuli vary in 

 potency. When the internal situation is weak only the most potent of the stimuli 

 can release the response. When the impulse is sufficiently strong almost any 

 objective stimulus will suffice. This organization is innate, and may appear in the 

 young long before maturity, since one young bird may open the beak to the con- 

 tact stimulus of another. The behavior of the young birds is likewise the expres- 

 sion of the internal conditions of hunger and certain objective stimuli. The range 

 of potential stimuli is again a function of the strength of the internal situation. 

 The regurgitating reflex is similarly the result of two independently variable 

 factors, namely, the fullness of the crop and the contact in the rear of the buccal 

 cavity which results from the thrust of the beak. Apparently this mechanism is 

 somewhat similar to the vomiting reflex in the human. 



The feeding activity develops from the previous activities of the cycle in two 

 ways. Its adequate stimulus the helpless birds in the nest is an obvious product 

 of the earlier activities. But an impulse to feed is also necessary to the act, and 

 this impulse is in part an outgrowth of the previous conditions, for birds will not 

 feed young unless they have participated in the cycle, and even then they will 

 feed only at a certain stage in the cycle. The very last few days of incubation 

 are quite essential to the impulse, for it will fail to develop if the birds do not 

 persist in their incubation duties to the end. The onset of the impulse is due to 

 the "milk-secretions" of the crop, and this is probably bound up physiologically 

 with the previous activities in a way analogous to the mammary-gland secretions 

 of mammals, though one must recognize that incubation and intra-uterine develop- 

 ment are radically different situations. Although the impulse originates in prior 

 conditions, its further development into functional potency is a result of its own 

 activity. 



INCUBATION. 



Incubation is likewise dependent upon two sets of conditions. The sensory 

 situation to which the bird responds is quite complex; it comprises the eggs, the 

 young, the nest in a given environment, and the various activities of the mate. 

 The "potency" of these factors is proved by the fact that their removal or alter- 

 ation will disrupt the act at times. For examples the reader is referred to Chapter 

 VII. The removal or breaking of one or both eggs, the death of one or both young, 

 the failure of the young to hatch, may each produce a disruption of the cycle. 

 The necessity of doing double duty, the sexual advances of the mate, the intro- 

 duction of a new mate, sexual union, a new nest, a new position for the nest, the 

 introduction of novel stimuli inducing fright, may each and all prevent a contin- 

 uance of incubation. The modus operandi of many of the stimuli is ambiguous. 

 We do not know the relative efficiency of sight and contact in the reaction to the 

 young and the eggs. It may be that neither the sight nor contact of the young 

 is the effective stimulus, but rather that the cessation of incubation is here due 



