AVIAN MIGRATION AND REPRODUCTION 701 



the preparatory phase and is prepared to begin the progressive phase. 



Although the present data favor the interpretation that the unin- 

 terrupted period of darkness is the effective stimulus of the short day, 

 the result of 16L-16D experiment indicates that this interpretation 

 must be accepted tentatively. If a 16-hr dark period in a 24-hr cycle 

 is the effective stimulus of a short day, why does not the preparatory 

 phase occur under a schedule of 16L-16D? Obviously, a 16-hr dark 

 period per se is not the only factor governing the reaction in the 

 preparatory phase. A long light period can negate the positive effect 

 of a long dark period. The ratio does not seem to be an important 

 factor, since 12L-12D gives a positive effect. Here again, the problem 

 of carry-over effects of the photoperiods may be involved; neverthe- 

 less it is clear that the data show some effect of the photoperiod. Thus, 

 although the evidence points to the dark period as the critical factor 

 in a short day, there also seems to be some relation between the photo- 

 period and the dark period, the net effect of which may be positive or 

 negative, depending on the length of the photoperiod. 



One final and significant observation must be noted, which bears 

 on the previous statement that the progressive phase proceeds spon- 

 taneously once the preparatory phase has been completed. A schedule 

 of 12L-12D is uniformly effective in the preparatory phase in late 

 July and August. Yet, when birds were treated with this schedule 

 beginning in October and were retained under the same conditions 

 until May, well past the natural occurrence of the premigratory 

 physiological state, or were treated with 20-hr photoperiods beginning 

 in December, many failed to respond. Birds held under natural day 

 lengths during fall and then treated with a schedule of 12L-12D 

 beginning in December responded well. If 12L-12D is an effective 

 schedule for preparation in late summer and is also an effective 

 schedule for the progressive phase, why did not the birds treated with 

 12L-12D beginning in October respond uniformly? To determine 

 whether the birds held until May had undergone the preparatory 

 phase, a few remaining birds were treated with 20-hr photoperiods 

 beginning in late May. They responded well, and hence, had com- 

 pleted the preparatory phase. The failure in these birds was appar- 

 ently in the progressive phase. There was also failure in the pre- 

 paratory phase, since some birds which were exposed to 20-hr 



