698 REPRODUCTION AND MIGRATION IN BIRDS 



ous daily effective photoperiodic schedules are required, further ex- 

 perimentation is needed. 



DISCUSSION AND SUMMARY 



When the results of all the experiments that have been described 

 so far are considered in relation to migratory behavior and reproduc- 

 tive rhythmicity, it is evident that day length in nature plays an im- 

 portant regulatory role in a number of ways. I shall consider migra- 

 tion and reproduction separately, since the data and the interpreta- 

 tions, although similar, are not identical for both. Also, in view of our 

 ignorance of the precise roles of light and darkness, I shall summarize 

 the regulation of migration and reproduction first in terms of day 

 length, with particular reference to what goes on in nature, and then 

 consider the roles of light and darkness. Finally, the question of an 

 inherent rhythm of the endocrine glands involved and the problem of 

 day length in relation to the timing of spring migration in equatorial 

 and transequatorial migrants and to breeding cycles in the tropics will 

 be discussed. 



Day Length and Migration 



The data available demonstrate that two phases or periods are in- 

 volved in the timing of spring migration. The first occurs in the late 

 summer and fall and is the preparatory phase; the second begins in 

 late fall and continues through the winter until the time of migration 

 in the spring and is the progressive phase. The preparatory phase is 

 prerequisite to the progressive phase. 



The preparatory phase in nature is apparently regulated by the 

 shorter day lengths of the fall. In the laboratory, this phase was in- 

 duced at different seasons of the year by subjecting birds to short days 

 (9L-15D). Twelve hours of light per day appears to be near the 

 threshold for the maximum day length which can act like a short day. 



Long days (16L-8D) inhibit the occurrence of the preparatory 

 phase and the appearance of the subsequent progressive phase. There- 

 fore, the short days of fall are actually regulating the occurrence of 

 the migratory behavior which appears six to seven months later. The 

 time required for the completion of the preparatory phase is six weeks 



