AVIAN MIGRATION AND REPRODUCTION 705 



vestigators, Blanchard (1941) and Marshall (1951, 1955) emphasize 

 the role of an internal rhythm. The studies reported here and the re- 

 cent work of Burger (1953), Farner and Wilson (1957), and Miller 

 (1954a,b, 1955) demonstrate the regulation of the entire annual 

 cycle by day length. Moreover, day length does not act like a trigger 

 that sets off an inherent cycle with an innate periodicity and ampli- 

 tude. Every phase of the reproductive and migratory cycles has been 

 shown to be influenced by day length — the time of occurrence, the 

 rate of development, the extent of development, and the duration of 

 the preparatory and progressive phases. Only one process appears to 

 be inherent, and that is the initiation and occurrence of regression of 

 the testes after a period of reproductive activity. However, the time of 

 regression and the extent of regression can be manipulated by day 

 length. More studies are needed to determine whether the process of 

 regression and the reorganization of the testis are independent of 

 environmental factors (Marshall, 1951). After regression and re- 

 organization of the testes have occurred, the cycle enters the prepara- 

 tory phase, which is clearly regulated by day length. 



Another possible inherent aspect of the cycle is the initiation of the 

 progressive phase. After birds have completed the preparatory phase, 

 the migratory response and spermatogenesis usually begin spontane- 

 ously. But they do not occur until after the completion of the prepara- 

 tory phase. 



If the initiation and occurrence of regression and the initiation of 

 the progressive phase are inherent processes and independent of ex- 

 ternal factors, their periodic occurrence may be called an inherent 

 rhythm. The term rhythm, however, would apply only to the alterna- 

 tion of these processes and could not be construed as implying the 

 time of occurrence, rate of development, ampUtude, or duration of the 

 various phases in the cycle, all of which have been clearly shown to 

 be influenced by day length. Further studies are needed that will per- 

 mit precise definhion of the physiological responses during the photo- 

 periods and the dark periods before we wifl be able to distinguish be- 

 tween inherent and independent reactions and reactions induced and 

 regulated by environmental factors. Admittedly, the present interpre- 

 tations which are based only on the manifest responses are not 

 satisfactory. 



