692 REPRODUCTION AND MIGRATION IN BIRDS 



It is impossible at this time to distinguish precisely between the 

 roles of light and darkness in the regulation of the gonadal and fat 

 responses. However, some things seem clear: (1) The total amount 

 of darkness in a 24-hr cycle is not the equivalent of the same amount 

 of darkness given in a single dose. (2) Short cycles within a 24-hr 

 period are equivalent to 24-hr cycles with regard to stimulation 

 (5L-1D and 20L-4D), but they are not equivalent with regard to 

 failure to stimulate, or "inhibition" (1L-2D and 8L-16D) as indicated 

 in (1). (3) With different effective schedules of light and darkness 

 within a 24-hr cycle, there is a difference in rate, extent, and duration 

 of response. (4) Whatever the roles of light and darkness, the daily 

 photoperiodic schedule is the critical external factor that results in 

 physiological "increments of response" which summate and are even- 

 tually manifested, or are readily observable and measurable as gonadal 

 growth, spermatogenesis, fat deposition, and increase in body weight. 



DAY LENGTH AND THE REFRACTORY PERIOD 



After a period of activity, the gonads regress spontaneously. In 

 nature, this occurs after the breeding season, some time in July and 

 August for most North Temperate species. During this period, long 

 days or increasing days cannot induce gonadal activity and, hence, 

 it has been called the refractory period. The natural termination of 

 this period varies with the species, but it occurs usually in October or 

 November. The refractory state can also be produced in the labora- 

 tory. Burger reviewed the status of the problem of regulation of repro- 

 ductive cycles in 1949 and aptly stated that "attention has been 

 focused too narrowly on the progressive phase of the reproductive 

 cycle." 



Since both the migratory and breeding cycles are closely in phase 

 with environmental conditions, it seems unlikely that the refractory 

 period, or the entire annual gonadal cycle, is regulated by fixed 

 inherent rhythms of the gonads or the pituitary which operate spon- 

 taneously as some investigators have suggested (Bissonnette, 1937; 

 Blanchard, 1941; Blanchard and Erickson, 1949; Marshall, 1951; 

 Wagner and Stresemann, 1950). 



