REPRODUCTIVE CYCLE. 97 



SEX DIFFERENCES. 



In the normal reproductive cycle many acts are performed by both birds. 

 Either bird may make advances, bill its own feathers, bow, coo, bill the other's 

 head, stamp and strike the feet, hug and neck, and, in one species, strive to obtain 

 the upper hold for mounting. Under certain conditions, bronze-wings of either 

 sex may stoop and receive the mate, and either may mount and exhibit all of 

 the characteristics of uniting behavior. Both participate in the search for and 

 selection of the nesting-site, and during the incubation period each bird takes its 

 turn in carrying straws and arranging them in the nest. Both incubate the eggs 

 and the young and both participate in the feeding of the young. 



Within this common group of activities, the sexes normally differentiate them- 

 selves in the characteristics of initiative, vigor, energy, and aggressiveness, and 

 in the frequency and completeness of many of the acts. The male, even in the 

 bronze-wings, is the more initiative and aggressive; his acts are performed with more 

 energy and vigor; the male bows, coos, stamps his feet, and mounts more frequently. 

 The female spends more time on the nest; she begins incubation somewhat earlier 

 than the male, continues it longer, and puts in more time per day. The mounting 

 activities of the female bronze-wing are infrequent, rather incomplete, and much 

 reduced in energy and life. 



Certain activities in the normal cycle are also peculiar to each sex. The activ- 

 ities normally manifested by the male alone are: Jealousy, display, charging and 

 driving, jumping over, presenting the open beak in billing, carrying straw for the 

 nest in the period prior to incubation, incubating by day only, and roosting away 

 from the nest at night. The acts peculiar to the female are retreating from the 

 driving male, insertion of the beak in billing, nest construction in the period before 

 incubation, egg-laying with its preliminary symptoms, and incubation by night. 



In the unnatural pairings of like sexes, however, it has been found that most 

 of those acts which are normally characteristic of the one sex may be exhibited 

 by the opposite sex. Under these conditions, a female may also charge and drive, 

 exhibit jealousy, jump over, present the open beak and receive that of the mate, 

 carry straw, incubate by day, and roost away from the nest at night. The only 

 masculine act not noted for the female was display. Under these unnatural con- 

 ditions the male may also retreat, insert the beak in billing, construct the nest, 

 and incubate at night. The egg-laying activities, from the nature of the case, 

 can not be (fully) manifested by males playing the feminine role, and possibly this 

 fact is due merely to the absence of the necessary stimulating intraorganic con- 

 ditions (ovulation). When either bird thus plays the part of the opposite sex it does 

 so rather unwillingly and under protest, as it were, and the resultant acts often 

 lack something of their normal vigor, finish, and completeness. Especially is this 

 true of many of the males forced to play the feminine role in copulation. 



So far as potentialities of kind of behavior are concerned, the two sexes are almost 

 on a par, and possibly we may correlate this with the fact that these birds usually 

 exhibit but little sex dimorphism. As a useful device for the comprehension of 

 this complex situation, the following conception may be suggested: There are 



