696 



REPRODUCTION AND MIGRATION IN BIRDS 



Fig. 3. Changes in body weight and fat deposition in two juncos for 

 the first cycle of short and long days. 



it seems more appropriate to call it the preparatory period or phase. 

 This period is then followed by the progressive phase. 



ROLE OF LIGHT AND DARKNESS IN REGULATION 

 OF REFRACTORY PERIOD 



Having established the requirement for short days during the pre- 

 paratory period, the next problem was the determination of the 

 effective part of the short days. Was it the short photoperiods or the 

 long dark periods? A variety of schedules was used during the natural 

 refractory period which was followed by subsequent treatment with 

 long days (20L-4D) beginning in December. The schedules used 

 and their effectiveness in inducing the preparatory phase are sum- 

 marized. 



