734 REPRODUCTION AND MIGRATION IN BIRDS 



(Benoit, Assenmacher, and Walter, 1950b); furthermore (1950a) 

 castration during the refractory period fails to cause hypertrophy of 

 the adenohypophysis and unilateral castration fails to cause the usual 

 hypertrophy of the intact testis. These observations suggest that the 

 site of refractoriness must be at, or above, the level of the hypophysis; 

 possibly it is hypothalamic. 



It is of interest to note that in many species the first-year birds are 

 also refractory in late summer and that the refractoriness terminates 

 at about the same time as in the adults. The only well-known excep- 

 tion appears to be that of Passer domesticiis in which the young 

 become susceptible to photoperiodic gonadal activation earlier in fall 

 (Riley, 1936; Davis, 1953). The question as to whether refractoriness 

 in first-year birds is the same as that of the adults which have com- 

 pleted a gonadal cycle is still unanswered. 



The basic cause of refractoriness is likewise still uncertain. Hypoth- 

 eses involving gonadal feedback mechanisms have been considered, 

 but attempts to obtain experimental support for them have thus far 

 been relatively unsuccessful. In our laboratory we were able to obtain 

 only a partial reduction of the photoperiodic response by testosterone 

 treatment of Zonotrichia leucophrys gam belli (Farner and Wilson, 

 unpublished data) and no apparent suppression with progesterone 

 (Laws and Farner, unpubhshed data). Burger (1949) was similarly 

 unsuccessful in treatment of Sturnus vulgaris with testosterone. On 

 the other hand, Kobayashi (1954b) has been able to reduce markedly 

 the photoperiodic responses in Zosterops japonica with simultaneous 

 treatment with either testosterone or estradiole. Of very substantial 

 interest is the demonstration by Bailey (1950) of an inhibition of 

 light-induced gonadal development by prolactin in Zonotrichia leuco- 

 phrys pugetensis. Possibly the normal sequence of events in the 

 adenohypophysis following photostimulation is an increased produc- 

 tion first of the gonadotropin (s) which stimulate testicular develop- 

 ment after which, possibly because of a gonadal feedback mechanism 

 or possibly because of a feedback mechanism operating at the level 

 of the hypothalamus and adenohypophysis, comes an increased pro- 

 duction of prolactin which could be responsible, directly or indirectly, 

 not only for the development of incubating and brooding behavior but 

 also for the development of refractoriness. In this conjunction it is 



