AVIAN MIGRATION AND REPRODUCTION 697 



Effective Schedules 

 8L-16D 12L-12D 



Ineffective Schedules 



4L-8D-4L-8D 5L-1D-5L-1D-5L-1D-5L-1D 



6L-6D-6L-6D 8L-7.25D-1.5L-7.25D 



These results demonstrated that it was not a short period of light 

 per se, nor the total amount of light and darkness in a given day, nor 

 the proportion of light to darkness in a short cycle that was effective. 

 Rather, it was a single period of darkness, 12 hr long, or longer, in a 

 24-hr cycle that was the effective part of a short day. A 16-hr dark 

 period was generally more effective than a 12-hr period, which seems 

 to be near the threshold for the minimum effective duration of the 

 dark period. 



The next question was whether a 16-hr dark period per se was the 

 effective stimulus, or whether a long single dose of light was inhibi- 

 tory. To answer this, birds were exposed to schedules of 16L-16D 

 and (1L-2D)' + IL + 16D during the preparatory period and then 

 treated with long days (20L-4D). Both schedules were ineffective 

 and corresponded to long days. That they were actually long days was 

 established by the response of birds to those same schedules during 

 the winter. The results suggest that a 16-hr dark period per se is not 

 the effective stimulus. There apparently is some relation between the 

 duration of the light period and the duration of the dark period, or 

 there is a single dose of light which is inhibitory irrespective of the 

 length of the dark period. Further studies are needed to explore these 

 alternatives. 



Another question of interest was whether a bird can "store" each 

 daily increment of response. If it could, what would the response be 

 to alternating short and long days? The schedule used was 8L-16D, 

 16L-8D. It turned out to be a long day during the preparatory period 

 and during the subsequent progressive period. This may have been 

 due to the fact that the bird was reacting to the schedule not on a daily 

 basis, but rather as 8D-8L and 16D-16L, both of which are known to 

 act like long days. To answer the question of whether the bird can 

 store a daily stimulus until a threshold is reached, or whether continu- 



