704 REPRODUCTION AND MIGRATION IN BIRDS 



to be 8.5 hr for the starling (Burger, 1949), about 8 or 9 hr for the 

 house sparrow (Bartholomew, 1949; Farner and Wilson, 1957), 9 

 hr for the slate-colored junco (Winn, 1950) and 10 hr for the golden- 

 crowned sparrow. In most of these studies, however, the effectiveness 

 of shorter day lengths was not explored. 



Once the preparatory phase has been completed, the progressive 

 phase is probably initiated spontaneously and proceeds at a rate which 

 is determined by the day length. Complete spermatogenesis was first 

 achieved in about 35 days under the longest days, 24L; 20L-4D; in 

 about 75 days with a schedule of 15.5L-8.5D; in about 150 days 

 with a 12L-12D schedule; and in about 166 days under natural day 

 lengths. 



In the experiment in which a schedule of 12L-12D was used begin- 

 ning in the fall, some of the birds did not initiate a migratory respon- 

 sive phase spontaneously, even though they had completed the 

 preparatory phase. With regard to the reproductive response, most of 

 the birds did not initiate the progressive phase spontaneously, even 

 though most had completed the preparatory phase. 



The duration of maximal reproductive activity is also regulated by 

 day length. The shortest durations were produced by long days, the 

 longest by days with 12-hr photoperiods. With a schedule of 12L- 

 12D begun in April, a few birds maintained reproductive activity 

 three to six times as long as the normal duration of about two months. 



Regression of reproductive activity occurs spontaneously, and the 

 time that it occurs is obviously under the control of day length. It can 

 also be induced by short days. After regression the birds enter the 

 preparatory phase, and a new cycle is ready to begin. 



Role of Light and Darkness in the Reproductive Cycle 



The conclusions and discussion given above in relation to the 

 migratory cycle are applicable here and need not be repeated. 



Inherent Rhythm and Migratory and Reproductive Periodicity 



Ever since the discovery that day length influences the reproductive 

 and migratory cycles, there have been suggestions that these cycles, in 

 whole or in part, are regulated by internal rhythms which are inde- 

 pendent of external factors such as day length. Among current in- 



