REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1139 



(Marshall, 1959). The light-induced gon- 

 adal response can be obtained again only 

 after the end of this so called "refractory 

 period." The refractory period plays an im- 

 portant role in the regulation of the gonadal, 

 migratory, and fat deposition cycles of 

 migratory birds (Wolfson, 1959a, b). Wolf- 

 son studied the effect of dift'erent photo- 

 l)eriods on the gonadal and fat deposition 

 cycles of Junco hyemalis and Zonotrichia 

 albicollis. In these birds, in the fall and 

 spring, large amounts of fat are deposited in 

 the subcutaneous and interperitoneal depots. 

 These fat dei)ositions are closely associated 

 with the migratoiy drive (Zugunruhe), and 

 account largely for the increase in body 

 weight at those seasons. 



Kobayashi ( 1954, 1957) proposed on the 

 basis of light-induced molt and gonadal 

 cycles in male and female canaries that the 

 refractory period reflects an increased secre- 

 tion of TSH at the expense of gonado- 

 troi)hin secretion. This hypothesis needs 

 further verification, however, for gonado- 

 trophins have been found in the anterior 

 pituitaries of drakes during the refractory 

 period. 



Fall migration will not be discussed be- 

 cause too little is known about the regula- 

 tion and physiologic conditions associated 

 with it. 



AVolfson (1959a, b) investigated the abil- 

 ity of various photoperiods to induce gon- 

 adal and fat de])osition in birds caught in 

 the fall and spring. These birds w^ere ex- 

 posed to light schedules of 9 L(ight) + 15 

 D(ark) hours; 12 L + 12 D; 15.5 L + 8.5 

 D ; 20 L + 4 D ; and 24 L and 9 L + 15 D ; 

 12 L -h 12 D ; 20 L + 4 D and 24 L, respec- 

 tively. The experiments showed: (1) the 

 rate of the gonadal and fat responses is a 

 reflection of photoperiod, the rate being 

 greater with longer photoperiods; (2) the 

 degree of response is greater with longer 

 photoperiods; (3) even under short photo- 

 periods (9 L ) , a response can be obtained ; 

 (4) the time interval between gonadal stim- 

 ulation and regression is smaller for the 

 longer photoperiods. 



Wolfson (1959a, b) formulated his sum- 

 mation hy})othesis on the basis of these re- 

 sults. The hypothesis is that the sum of the 

 photoperiods and not the changes in day- 



light to which the birds are subjected de- 

 termines the response. 



Subsequent experiments were designed to 

 test the importance of the dark period by 

 interrupting the dark periods by short light 

 l)eriods. These experiments tested the hy- 

 l)othesis of Jenner and Engels (1952) and 

 Kirkpatrick and Leopold (1952) that the 

 dark period has a positive effect. Wolfson 

 compared the effect of 8 L + 7.25 D to 

 1.5 L + 7.25 D with 8 L + 8 D and found 

 no difference between the treatments. Thus, 

 8 + 1.5 L in 24 hours was as effective as 

 16 L per 24 hours, whereas previous experi- 

 ments had shown that 9.5 L in one dose per 

 24 hours was relatively ineffective. This 

 evidence made a positive effect of dark peri- 

 ods seem unlikely. 



Experiments were then designed to test 

 the effectiveness of different dark periods in 

 breaking up the refractory period. Ex- 

 posure to darkness had been used for 

 centuries by the Dutch to interrupt the 

 refractory period (Rowan, 1938b; Damste, 

 1947). The experiments by Wolfson demon- 

 strated that 12 hours of uninterrupted dark- 

 ness per 24 hours were required to abolish 

 the refractoriness to light. However, 12 

 hours darkness alone does not seem to be 

 sufficient to abolish refractoriness, because 

 on 16L + 16D the refractory period is not 

 broken. This suggests that the photoperiod 

 may also have an effect. 



On the basis of these results, the regula- 

 tion of gonadal and migratory cycles can 

 be tentatively explained for birds of the 

 temperate zone. For the present dis- 

 cussion the gonadal and migratory (fat 

 deposition) cycles will be regarded as one, 

 although there are some quantitative dif- 

 ferences with respect to the rate of response. 

 Nonmigratory species and races do not show 

 the fat deposition cycles shown by migra- 

 tory races of the same species; the discus- 

 sion here, therefore, is concerned only with 

 the gonadal cycles. In late summer and early 

 fall the birds enter what Wolfson calls a 

 preparatory phase (similar to Marshall's 

 (1959) regeneration phase). Birds need ex- 

 posure to at least 12 hours of darkness per 

 24 hours to enter this physiologic state. 

 After exposure to such dark periods for a 

 certain length of time, depending on the 



