AVIAN MIGRATION AND REPRODUCTION 713 



When day length is suggested as a regulatory factor at all latitudes, it 

 is not meant to imply that all birds will react to it in precisely the 

 same manner. 



On the basis of the more recent findings presented in this paper, it 

 is clear that the day length conditions in the Tropics could regulate 

 the gonadal cycles in a variety of ways, and it is not reasonable to 

 think of light only in relation to the progressive phase. Our experi- 

 ments indicate that day length not only regulates the time of initiation 

 of the progressive phase of the testis, but also the rate and amplitude 

 of the progressive phase, the duration of maximum activity, the time 

 of initiation, rate, and extent of regression, and the duration of the 

 preparatory phase. There is no reason to believe that all these aspects 

 will be regulated by day length in precisely the same manner in all 

 species. Nor must all these aspects be regulated by day length. Regula- 

 tion of a few is all that is needed for day length to be a primary factor 

 in the timing of breeding seasons. It should be recalled that races of 

 the same North Temperate species responded differently to identical 

 experimental conditions of day length. When tropical species have 

 been studied experimentally as intensively as North Temperate spe- 

 cies, they will surely show differences in their response to light and 

 dark periods. Moreover, other environmental factors, such as rainfall, 

 diet, and psychic factors, may prove to be important, with different de- 

 grees of effectiveness, in modification of the preparatory, progressive, 

 and regressive phases. Only day length has been shown so far to be a 

 primary regulatory factor, and its relation to gametogenesis and repro- 

 ductive rhythmicity in tropical species remains to be determined by 

 extensive experimental studies. 



REFERENCES 



Baker, J. R. 1938. The relation between latitude and breeding seasons in birds. 

 Proc. Zool. Soc. London, A 1 08. 557-82. 



Baker, J. R., and R. M. Ranson. 1938. The breeding seasons of southern hemi- 

 sphere birds in the northern hemisphere. Proc. Zool. Soc. London, A108, 

 101-41. 



Bartholomew, G. A. 1949. The effect of light intensity and day length on 

 reproduction in the English sparrow. Bull. Museum Comp. Zool. Harvard 

 Univ., 101, 433-76. 



