CONTROL OF ANNUAL GONADAL CYCLES 737 



effect of environmental temperature on Sturnus vulgaris (Burger, 

 1948), Zonotrichia leucophrys gambelii (Farner and Mewaldt, 1952; 

 Farner and Wilson, 1957b), and Jiinco hyemalis (Engels and Jenner, 

 1956) show that environmental temperature may have a small but 

 significant modifying role. 



Possibly we can then envision a scheme in which the basic timer is 

 the genetically fixed photoperiodic response mechanism, with the 

 temporal and spatial differences in the time of attainment of full 

 sexual development being attributable to "psychic" factors, differences 

 in environmental temperature, and possibly other modifying influ- 

 ences such as nutritional factors (Bissonnette, 1932c, 1933a). Pre- 

 sumably the potential magnitude of the modifying effects varies 

 among different species. Such a scheme could account adequately for 

 the well-known irregular annual differences in rates of gonadal de- 

 velopment such as noted by Marshall (1949b). 



In the vast literature on reproductive cycles in birds the suggestion 

 of innate cycles has appeared frequently (see, for example, Marshall, 

 1951), If "innate cycle" is intended to mean a cyclic phenomenon 

 with a precision and frequency characteristic of most avian reproduc- 

 tive activity, I am of the definite opinion that there exists at present 

 no unequivocal evidence that it occurs in any species. On the other 

 hand, there is evidence that in some species there may be an innate 

 tendency for gonadal development in a constant environment. The 

 most striking example of this comes from the series of testicular 

 developments and regressions observed by Benoit, Assenmacher, and 

 Brard (1955, 1956) in drakes placed in either constant darkness or 

 in constant light. In both instances the "cycles" were irregular both 

 in amplitude and frequency. Miller (1955) has shown that Zono- 

 trichia leucophrys nuttalli held on midwinter daily photoperiods of 

 10 hr underwent a considerable testicular development by early May, 

 several months later than the normal time for the attainment of the 

 corresponding degree of development under natural conditions. Simi- 

 lar treatment of Zonotrichia atricapilla caused a slight testicular 

 development by early July when maximum development is attained 

 with natural daily photoperiods. In our laboratory (Laws and Farner, 

 unpublished data) we found no testicular development in Zonotrichia 

 leucophrys gambelii held in 8-hr daily photoperiods from midwinter 



