688 REPRODUCTION AND MIGRATION IN BIRDS 



pointed to the duration of the dark period in a 24-hr cycle as the 

 regulatory factor in the daily schedule, but their experiments, which 

 involved interrupting the dark period with a brief period of light, did 

 not rule out the proportion of light to darkness as the effective daily 

 stimulus. For example, in the schedules used, 8.25L-7D-1.75L-7D 

 (Jenner and Engels, 1952) and 9L-7D-1L-7D (Kirkpatrick and 

 Leopold, 1952) the first cycle of light and darkness in the 24-hr 

 period was close to a 1:1 ratio and the light period was longer. This 

 could have been the effective cycle of the 24-hr period. The second 

 cycle, with its much greater proportion of darkness might not have 

 been stimulatory. The next experiments in our laboratory involved 

 the use of an interrupted-night schedule and a related one to examine 

 the role of the ratio of light to darkness. The schedules were as 

 follows: 8L-7.25D-1.5L-7.25D and 8L-8D. In the first schedule, the 

 16-hr dark period was interrupted with 1.5 hr of light. In the second, 

 the 8L-8D simulated the 8L-7.25D part of the interrupted night 

 schedule. 



The birds responded similarly under both treatments. A preliminary 

 analysis of the data indicates that the rate of response was close to 

 that in birds exposed to 15.5L-8.5D. Hence, both of these schedules 

 acted like the long days (15.5L, 20L) of the earlier series of experi- 

 ments. Yet, the amount of light in a 24-hr period was only 9.5 hr 

 in the interrupted night schedule. In the 8L-8D schedule, the amount 

 of light alternated from 8 to 1 6 hr per 24-hr period. With a schedule 

 of 8 hr or 9.5 hr of light per day in one dose in a 24-hr cycle, birds 

 do not respond in eight or ten weeks. 



The results of these experiments confirmed the work of Kirkpatrick 

 and Leopold (1952) and Jenner and Engels (1952) and pointed to 

 the length of the night or dark period in a 24-hr cycle as the regula- 

 tory part of light-darkness schedules which induce photoperiodic 

 responses. An alternative explanation was put forth by Farner et al. 

 (1953a,b). They suggested that the daily dark period per se has no 

 positive function; rather, there is a persistent carry-over period which 

 follows the end of the photoperiod, and the effective part of a photo- 

 periodic schedule is the photoperiod plus the carry-over period. They 

 envisioned the carry-over period as having a duration of a fraction 

 of an hour to several hours and as a probable function of the duration 



