718 REPRODUCTION AND MIGRATION IN BIRDS 



interrelated annual cycles all of which are probably photoperiodically 

 timed in one way or another. Among these cycles are an annual 

 metabolic cycle, which is expressed patently in premigratory fat de- 

 position, and the migratory cycle itself. Our knowledge of the control 

 of these two cycles has been summarized by Wallgren ( 1954), Farner 

 (1955), Weise (1956), Wagner (1956), Rautenberg (1957), and 

 King (1957). Unquestionably also the molting cycle is photoperiodi- 

 cally controlled although perhaps quite indirectly. At present it ap- 

 pears not possible to construct a satisfactory rational hypothesis for a 

 control system. 



Of the photoperiodically controlled annual cycles, the gonadal 

 cycles, and, more particularly, the testicular cycles have received the 

 greatest attention and are consequently best understood. The seasonal 

 cycle in avian testicular size was apparently known to Aristotle 

 (Etzold, 1891; Disselhorst, 1908). More than a half century ago 

 Etzold (1891) and Schafer (1907) suggested a possible functional 

 relation between day length and the vernal testicular development. In 

 a practical way, however, photoperiodic control of testicular cycles 

 was actually practiced artificially in very early times in the Nether- 

 lands (Damste, 1947) and in Japan (Miyazaki, 1934) in order to 

 obtain unseasonal singing in males. The pioneer investigations, how- 

 ever, were those of Rowan (1925, 1926, 1938b) with Junco hyemalis 

 and those of Bissonnette (1930, 1931a) with Sturnus vulgaris. Since 

 these early studies sufficient information has been accumulated to 

 indicate that photoperiodic controls of gonadal cycles must be opera- 

 tive in at least 27 species among 12 families of birds (Table I). 

 Among the more recent reviews of this phenomenon are those of 

 Burger (1949), Galgano and Mazzi (1951), Hammond (1954), and 

 AschofT (1955). 



NATURE OF TESTICULAR GROWTH 



It has been possible to develop a more satisfactory quantitative ap- 

 proach in our investigations with Zonotrichia leucophrys gambelii by 

 recognizing the form of the growth curve of the testes (Farner and 

 Wilson, 1957a,b). When males are subjected to constant daily photo- 

 periods of stimulatory duration, the testes increase in weight approxi- 



