TESTES OF TRANSEQUATORIAL MIGRANTS 763 



The results of the first experiments (Table I), indicate a photo- 

 refractory phase which is maintained for at least SVi months by 14-hr 

 photoperiods (group A). Termination of this photo refractory phase 

 evidently was brought about by a prolonged exposure to 10-hr photo- 

 periods, followed by full recrudescence of the testes on 14-hr photo- 

 periods (group B). Constant 12-hr photoperiods not only permitted 

 passage from the photorefractory to the photoreceptive phase, but also 

 permitted (stimulated?) a slow recrudescence of the testes (group C) 

 at least to the beginning of production of secondary spermatocytes 

 and also to the production of sufficient sex hormone to cause pigmen- 

 tation of the beak. 



In later experiments (Table II), attempts were made to define 

 further the conditions under which the photorefractory phase might 

 be terminated and testicular recrudescence permitted or stimulated. 

 Constant 10-hr photoperiods apparently retarded production of sex 

 hormone at least until June 13 (group D) but obviously released the 

 birds from the photorefractory phase and permitted some regrowth 

 of the testes. Eight weeks exposure to 10-hr photoperiods beginning 

 October 1, followed by 14-hr photoperiods (group E), brought about 

 beak pigmentation before the end of February and maintained this 

 pigmentation for some months. Similarly, the combination of 8 weeks 

 exposure to 12-hr photoperiods beginning October 1 and then 14-hr 

 photoperiods (group F) evidently released photorefractoriness and 

 permitted eventually full recrudescence of the testes, with sufficient 

 production of sex hormone by mid-April to produce blackening of the 

 beak. In a final experiment photorefractoriness was apparently again 

 demonstrated, no beak pigmentation developing after 8 months ex- 

 posure to 16-hr photoperiods (group G). 



CONCLUSIONS 



These experiments demonstrate that a photoperiodic response does 

 occur in this transequatorial migrant. Just as in North Temperate 

 Zone species (cf. Burger. 1949), the postbreeding season is character- 

 ized by a photorefractoriness which is maintained by continuously 

 long photoperiods (groups A, G), but which is broken by a period of 

 short photoperiods (groups B, E, F). On continued 10-hr photo- 



