690 REPRODUCTION AND MIGRATION IN BIRDS 



was inhibitory (1L-2D), and since the total amounts of light and 

 darkness per day (8L and 16D) were also inhibitory, the positive 

 results ruled out both of these as effective aspects of the daily photo- 

 periodic cycle. They pointed instead to the effectiveness of small 

 amounts of light per day when there was no long period of darkness 

 or to the existence of carry-over periods, depending on one's inter- 

 pretation. 



To examine further the possible role of a long period of darkness 

 as an inhibitory factor, the pertinent experiments in the literature 

 were examined and another experiment was performed in our labora- 

 tory. The experiments in the literature (Burger et al., 1942; Farner 

 et al., 1953a,b; Jenner and Engels, 1952; Kirkpatrick, 1955; Kirk- 

 patrick and Leopold, 1952, 1953) showed that in most cases birds 

 responded to "light periods" given either as flashes of light or when 

 broken up with dark periods, as long as there was no single long dark 

 period of about 14 hr duration in a 24-hr period. There were some 

 exceptions. In some cases, the flashing "schedules" were not effective, 

 for example, 5 sec L-15 sec D and 0.1 sec L-5 sec D (Burger et al., 

 1952). In the experiments of Farner et al. (1953-a,b), the response 

 was poor in some cases, but only with a schedule of 12.2L-1 1.8D was 

 there a clear-cut failure to respond (9 out of 12 birds) when the dura- 

 tion of a continuous dark period was less than 14 hr. The other 

 experiments that they reported and interpreted in terms of carry-over 

 also had no continuous dark periods long enough to inhibit the light 

 periods. The experiments in the literature seemed to me to favor the 

 interpretation of a long period of darkness as inhibitory, but there 

 was also a basis for a carry-over effect of the photoperiod. 



To test the effect of an inhibitory period of darkness after an 

 effective series of light periods, the following schedule was used in 

 the next experiment in our laboratory: (1L-0.25D)'-1L-14.25D. 

 This schedule provided eight 1-hr periods of light as in the 1L-2D 

 schedule and a similar total of 8 hr of light in 24 hr. It differed in 

 having only 15-min dark periods interspersed among the 1-hr light 

 periods and in having one long continuous period of darkness that 

 was known to be inhibitory. The birds failed to respond, and this 

 schedule not only failed to initiate a response, but also failed to main- 

 tain the reproductive condition of some birds which were already 



