AVIAN MIGRATION AND REPRODUCTION 693 



Juncos in nature experience a reduction in day length during the 

 summer and fall, and it was plausible that decreasing day lengths, or 

 short days, or long nights, could regulate the duration of the refractory 

 period. Burger (1947) and Bissonnette (unpublished data, cited by 

 Burger, 1947) showed that treatment with long days in the starling 

 eventually induced a refractory period which could only be dissipated 

 by treatment with short days. Miller (1948, 1951) found that treat- 

 ment with long days begun in the fall in the migratory golden-crowned 

 sparrow {Zonotrichia atricapilla) prevented the occurrence of gonadal 

 growth at the normal time in the spring. 



To determine the effect of day length on the refractory period in 

 the junco and white-throated sparrow, birds whose cycles had been 

 accelerated by exposure to 20 hr of light per day and which were in 

 a refractory period were treated with short days beginning July 14. 

 One group was exposed to 9 hr of light per day, the other to 12 hr 

 per day. In addition, there was a group retained under natural day 

 lengths. After six weeks of this treatment they were exposed to long 

 days (20L-4D) beginning August 27. The experiment terminated 

 on October 22. The juncos that were pretreated with 9-hr and 12-hr 

 photoperiods showed an excellent response, whereas the birds pre- 

 treated with natural day lengths did not (Wolfson, 1952b). The 

 response in the white-throated sparrows was similar, but fewer indi- 

 viduals responded in the 12-hr group (Wolfson, unpublished data). 

 This experiment was repeated again in the fall during the natural 

 refractory period, and the results were generally similar (Wolfson and 

 Shank, unpublished data). In a related group of experiments, birds 

 were exposed to 20-hr photoperiods for a maximum of 6 months 

 beginning in the refractory period (October and early November) to 

 test the effect of a strong stimulatory photoperiod. With few excep- 

 tions, the birds failed to respond, even at the normal time of the 

 gonadal response in the spring (Wolfson, 1952b). 



The results of these experiments showed that the duration of the 

 refractory period was regulated by day length. Short days hastened 

 the termination of the refractory period, whereas long days prolonged 

 it. Whether the effective part of the photoperiodic cycle was the 

 period of light or the period of darkness, or a relation between them. 



