684 REPRODUCTION AND MIGRATION IN BIRDS 



theory, the significant results were (a) that constant day lengths, even 

 when as short as 9 hr and without increase from the shortest winter 

 days to which the birds are subjected in nature, induced the migratory 

 state and (b) that there is a relation between length of the day and 

 rate of response. 



The interpretation of the results of these experiments and the 

 earlier ones of Rowan and Wolfson was as follows (Wolfson, 1952a). 

 The daily photoperiod, whether constant or increasing gradually, 

 induces an increment of physiological change. The magnitude of the 

 increment is proportional to the daily photoperiod, and it appears to 

 be at maximum at 16 hr in a 24-hr cycle. The minimum is not known, 

 but it is less than 9 hr. The daily increments of physiological change 

 siimmate and eventually reach a threshold, at which time the responses 

 are manifested. During the period of summation, or in induction 

 period, no marked manifestations of the daily physiological increments 

 were evident. 



On the basis of the results obtained in the junco and the interpreta- 

 tion given above, day length could regulate the onset of migration in 

 birds wintering in the Tropics and in the Southern Hemisphere. Even 

 though the days are constant on the equator and are decreasing in the 

 Southern Hemisphere after December 21, they could be well above 

 the length of the daily effective photoperiod for the birds wintering 

 there, judging from the studies of North Temperate species. This 

 matter will be discussed later at greater length. 



DAY LENGTH AND DURATION OF MIGRATORY 

 AND REPRODUCTIVE RESPONSES 



This first series of experiments demonstrated a relation between 

 the duration of the photoperiod in a 24-hr cycle and the time of 

 initiation of the fat and gonadal responses. Was there also a relation 

 between the daily photoperiod and the maintenance or duration of 

 these responses once they have begun? If so, then the daily photo- 

 period would be a highly significant factor in the regulation of the 

 entire annual cycle and not just the initiation of spring migration and 

 the breeding cycle. Some relevant data were available from the 

 previous series of experiments, but a new series of experiments was 



