764 REPRODUCTION AND MIGRATION IN BIRDS 



periods (group D) there occurs eventually at least a slight recrudes- 

 cence of the testes, just as, for example, in crowned sparrows (Miller, 

 1955) and juncos (Engels and Jenner, 1956). Recovery from photo- 

 refractoriness while exposed to 12-hr photoperiods (group C) has 

 been reported also for juncos (Wolfson, 1952). 



The deviations of the response (groups E, F) from previously 

 established patterns are those which would be made necessary by the 

 "winter" sojourn in the Southern Hemisphere spring and summer. 

 After exposure to only 10 hr of light per day for as long a time as 8 

 weeks, a minimum of at least 11 weeks on 14-hr days was required to 

 reach a level of sex hormone production manifested by beginning of 

 beak pigmentation (group E). This level almost certainly coincides 

 with that stage of spermatogenesis when most primary spermatocytes 

 are in the process of division. It would be expected, I am confident, 

 that, under lighting schedules such as given to the bobolinks in group 

 E, testes of juncos, crowned sparrows, and similar North Temperate 

 Zone migrants would in 1 1 weeks not only have attained this stage, 

 but actually would also have reached maximum development, with 

 mature sperm, at a much earlier time. 



The results with group F, compared to those with group E, seem 

 to be especially significant in interpreting this peculiarity of the bobo- 

 link response. If full recovery from photorefractoriness actually had 

 occurred during 8 weeks both on 10-hr and on 12-hr photoperiods, 

 then one would expect that, on the 14-hr photoperiods that followed, 

 beak pigmentation would develop at about the same time in both 

 groups. Instead, the group F birds were 6 to 8 weeks behind the 

 group E birds in showing this response. It appears to me that the most 

 reasonable explanation of this difference is that complete release from 

 photorefractoriness had not in fact occurred during the period of 

 short days, that transition from the photorefractory phase to a photo- 

 receptive phase took place during the 14-hr photoperiods and oc- 

 curred earlier in the case of those birds that previously had been ex- 

 posed to the shorter photoperiod. Consideration of Fig. 1 indicates 

 that recovery from photorefractoriness necessarily must occur in na- 

 ture during or after exposure to long days following an earlier period 

 of shorter days. The results with our groups E and F show that the 



