702 REPRODUCTION AND MIGRATION IN BIRDS 



photoperiods beginning in late December, also failed to respond. The 

 differences in "preparation" and "responsiveness" between the birds 

 treated in summer and fall may stem from the differences in the his- 

 tory of the birds before the experiments. Whatever the explanation, 

 what is significant is the observation that in some birds exposure to a 

 schedule of 12L-12D in the fall modified the day length requirements 

 of the progressive phase, or changed the requirements for the spon- 

 taneous initiation of the progressive phase. 



In the progressive phase, both the light and dark periods have been 

 suggested as the controlling part of the photoperiodic cycle. Jenner 

 and Engels (1952), and Ki'rkpatrick and Leopold (1952, 1953) and 

 Kirkpatrick (1955) state that the dark period is the critical factor. 

 Farner et al. (1953a,b) state that the effective stimulus is the light 

 period and the carry-over effects of the light periods. In these studies 

 only the reproductive response was used, but the ideas are applicable 

 to the migratory response and hence are discussed here. 



The results of our studies of the migratory response can be inter- 

 preted to support either hypothesis. The available data do not permit 

 a clear-cut choice. Perhaps they do not because both light and dark- 

 ness play important roles in the daily photoperiodic schedule. The 

 results of the experiments using the schedules 16L-16D and 12L- 

 16D seem to me to favor the light period (and any carry-over 

 periods) as the effective daily stimulus. It should be noted here also 

 that birds exposed to continuous light responded well and as rapidly 

 as birds exposed to schedules of 20L-4D and 15.5L-8.5D. Whatever 

 the effective stimulus is, the daily responses to it summate and ulti- 

 mately induce the migratory response. 



An interesting difference between the "light and dark" reactions of 

 the preparatory and the progressive phases is that in the preparatory 

 phase, where the dark period seems to be the critical factor, the effects 

 of shorter dark periods in a 24-hr cycle do not summate to give an 

 effective daily stimulus. In the progressive phase, where the light ap- 

 pears to be the critical factor, the effects of short light periods do 

 summate to give an effective daily stimulus when there is no inhibitory 

 duration of darkness in a 24-hr cycle. In the preparatory phase a long 

 light period per day appears to be inhibitory, whereas in the progres- 

 sive phase, a long dark period per day appears to be inhibitory. In 



