686 REPRODUCTION AND MIGRATION IN BIRDS 



of the birds in the 12-hr group retained their fat deposits for many 

 months and failed to molt. 



The results of the first series of experiments showed that the daily 

 photoperiod determined the time at which the response begins. The 

 results of the second series demonstrated that the daily photoperiod 

 regulates also (a) the rate at which the response proceeds, (b) the 

 degree of response, and (c) the duration of the active phase of the 

 gonadal and fat cycles (or the time of gonadal regression). A few 

 data suggest that the extent of gonadal regression is also influenced 

 by the daily photoperiod. 



ROLE OF LIGHT AND DARKNESS IN INITIATION OF 

 MIGRATORY AND REPRODUCTIVE RESPONSES 



At about the time the second series of experiments was concluded, 

 Kirkpatrick and Leopold (1952, 1953) and Jenner and Engels 

 (1952) demonstrated the importance of the dark period in the gonadal 

 response to the daily cycle of light and darkness. In the studies in our 

 laboratory described above, none of the experiments had been designed 

 to distinguish between the roles of light and darkness. Hence, further 

 experiments were undertaken to determine the role of the dark period 

 in relation to the summation hypothesis. 



If the effective stimulus is the amount of light that a bird receives 

 in a day, or in a period of time, then birds receiving 20 hr of light 

 per day in one dose or in four equal doses of 5 hr should respond 

 equally well. In both cases the birds would receive the same total 

 amount of light per 24-hr period. They would also receive the same 

 ratio of light to darkness. The only basic difference would be the 

 length of each cycle. Hence, in the first experiment the birds were 

 subjected to 5-hr photoperiods followed by 1-hr dark periods (5L-1D) 

 (Wolfson, 1953). 



The results demonstrated clearly that a 6-hr cycle of 5L-1D was 

 strongly stimulatory for the gonadal and fat responses. The extent 

 and rapidity of response was similar to that in birds on a 20L-4D 

 schedule, with a tendency toward a greater fat response and a greater 

 reproductive response in the females. The changes in body weight 



