AVIAN MIGRATION AND REPRODUCTION 703 



both phases, the data show clearly that the 24-hr cycle is highly 

 significant. It is possible that an inherent 24-hr rhythm or other innate 

 rhythms in the bird are related to the migratory and reproductive re- 

 sponses to photoperiodic schedules, and this must be explored in 

 future studies. 



Day Length and Reproduction 



The regulation of the reproductive cycle in the male also occurs in 

 two phases, the preparatory and the progressive. The preparatory 

 phase occurs in late summer and fall, the progressive phase in late 

 fall, winter, and spring. As with migration, the preparatory phase is 

 prerequisite to the progressive phase. 



The regulation of the preparatory phase by day length is the same 

 as that for migration. There are some differences, however, in the 

 responsive phase. A 9-hr photoperiod per day induced migration at a 

 slow rate. However, only one junco showed a good testicular response 

 (complete spermatogenesis, but not maximum size) under a 9L-15D 

 schedule and that was almost twelve months after the experiment be- 

 gan (December 4 to November 23). The other birds which were 

 sacrificed during the course of the experiment, showed only a slight 

 development. Hence, a short photoperiod is much more retarding in 

 the progressive phase of the reproductive cycle. 



The minimum photoperiod that will induce a reproductive response 

 varies with the species, the duration of the experiment, and what is 

 defined as a response. House sparrows showed maximum testicular 

 development on constant 10-hr photoperiods per day (Bartholomew, 

 1949). In the starling, 9.5-hr photoperiods per day were effective for 

 complete development (Burger, 1953). In a more carefully controlled 

 experiment. Miller (1955) showed that a 10-hr photoperiod was not 

 sufficient for complete development of the testes in the golden- 

 crowned sparrow, a migratory form, but it was sufficient for complete 

 development in a nonmigratory form (nuttalU) of the white-crowned 

 sparrow (Zonotrichia leucophrys). Millers experiment terminated on 

 September 20, which was about two months sooner than the time it 

 took the junco to show a good (but not complete) response with a 

 constant 9-hr photoperiod per day (Winn, 1950, Wolfson, 1952a). 

 The minimum effective day length for some testicular response seems 



