REPRODUCTIVE CYCLE. 103 



to the stimulus of an unrelieved crop resulting from their absence. Neither can 

 any assertions be made as to the "relative" potency of the various objective factors. 



The existence of an "impulse" as an essential condition of incubation is like- 

 wise obvious. Mating birds will not incubate at all times, even though all the 

 objective conditions are present. If the objective stimuli were the only decisive 

 factors, incubation should persist until the young desert the nest. A pair may enter 

 the incubation stage and then desist without any objective defect. Also, incubation 

 may persist for the normal period when most of the objective conditions are absent. 

 Rich food may terminate the act before its normal end, and the author has inter- 

 preted this fact to mean that the disposition to incubate finds its primary root 

 in the "exhaustion" resulting from the previous sexual behavior and that the 

 disposition will tend to persist until recuperation occurs. On this basis rich food 

 will hasten recuperation and shorten the duration of the period. 



The great variety of results (Chapter VII) may be explained by supposing that 

 the various conditioning factors vary independently of each other in potency. 

 If the disposition be "weak," the removal of any one of the objective factors will 

 disrupt the act; when the disposition is "strong," the same objective defect will 

 be impotent and disruption will result only from a number of deficiencies. Given 

 an impulse at its maximum strength, incubation will persist in the absence of 

 practically all of the objective factors. 



Incubation is a result of the previous mating and nesting activities. The 

 causal nexus is mediated in two ways. The previous activities furnish the objec- 

 tive stimuli essential to the act, viz, the mate, the eggs, and the nest in a partic- 

 ular environment. Likewise the disposition to sit is a physiological outgrowth 

 of these former acts. Numerous lines of evidence may be adduced in support of 

 this latter proposition. There is no case of a bird manifesting a disposition to 

 sit without some indication of previous sexual activity. The male of the blond 

 X white ring pair (X-W 1} was slow in his sexual response and, as a consequence, 

 began incubation 4 days late. All of the objective conditions essential to incuba- 

 tion were present, but no disposition was aroused until the usual period of sexual 

 activity was completed. Normally the birds do not always begin incubation 

 coincident with laying. As to the relative efficiency of the various constituents 

 of the sexual 'and nesting activities in contributing to the development of the 

 incubation impulse, no very confident assertions can be made. Probably "every 

 phase" of these prior activities contributes somewhat to the result. Undoubtedly 

 both the prior dispositions, as well as their behavior manifestations, are effective. 

 On the one hand, the impulse to sit sometimes develops when the sexual and nest- 

 ing impulses are denied any adequate expression, and, on the other hand, deficient 

 expression often results in a very "tardy" development of incubation. Several 

 pertinent facts may be mentioned. The nesting activities, in these conditions 

 of close confinement, are, for the wild species at least, far from normal. Females 

 may incubate without having laid eggs. Both males and females may incubate 

 when they were forced to assume the r61e of the opposite sex in mating and nesting. 



