REPRODUCTIVE CYCLE. 107 



The impulses arise in the two birds somewhat independently of each other. 

 Synchronization of the two cycles is effected in two ways. The two birds stimu- 

 late each other to some extent, and each bird will remain in a state of sexual readi- 

 ness for some time when interaction is absent. The initiation of the subsequent 

 stages of the cycle is timed primarily in reference to the beginning of sexual inter- 

 action rather than with the onset of the sexual impulse. This conception involves 

 the assumption that many of the preliminary courting activities serve in part to 

 attract, interest, and sexually excite the mate. The author is definitely of the 

 opinion that the display activities serve this function. Necessarily the exact 

 function of each act and the mechanism whereby the result is achieved must 

 remain somewhat speculative. There are four possible functions of the activities : 

 (a) to excite the sexual impulse in the mate; (6) self-excitation; (c) to arouse in 

 the mate the attitudes and behavior necessary to coition; or, (d) to serve as motor 

 outlets of energy incidental to the sexual act. A few speculative suggestions may 

 be offered. 



(A} The act of "billing" probably serves the first purpose. This act of taking 

 the beak of another bird into the mouth occurs in both courting and feeding. The 

 act per se is nearly, if not quite, identical in the two cases. The differences lie 

 in the situation in which the impulse develops, the nature of the stimulus, and the 

 functional result. In many organisms there seems to be an intimate functional 

 relation between the sexual activities and those processes involved in the care 

 and feeding of the young. The sexual activities and their physiological conse- 

 quents stimulate and arouse the food secretions. Nursing may suppress the 

 sexual impulse. The stimulation of the sensory regions involved in the feeding 

 and care of the young tends to excite the sexual passion. The vigorous thrust 

 of the beak stimulates regurgitation with one internal situation, but arouses passion 

 when the bird is sexually disposed. Though the primary result of the act in court- 

 ing is sexual excitement, yet feeding does occasionally occur as an incidental by- 

 product when the internal conditions are such as to favor it. In the later stages 

 of the cycle the primary purpose of the act is to feed the young, and likewise we 

 might expect at this time a secondary result of sex-excitement if the conditions 

 are at all favorable. Such conditions are not present during the first week or so, 

 while close incubation is the rule; for, as we have seen, incubation and sexual 

 activity are mutually exclusive functions. As incubation gradually terminates, 

 however, this possibility exists, and as a matter of fact sexual activity frequently 

 appears at this point. This conception of an incidental sex stimulation resulting 

 from the feeding activities will thus explain the very general tendency toward an 

 immediate renewal of the cycle and the synchronous appearance of the sexual 

 impulses in the two birds. 



The conception presented above is further supported by an observation fur- 

 nished by Dr. Riddle : 



"Two male blond rings had been paired and given eggs to incubate. The young were 

 hatched and fed, and the pair were again given eggs in due time. The previous young had 



