Abstract
Male European starlings held under constant 12-h days (LD 12:12) undergo circannual rhythms of gonadal maturation and molt. This study exam ined the temporal relationship of the different physiological phases of these rhythms and the associated endocrinology The majority of males and about half of the females in this study showed evidence of rhythmicity in gonadal matura tion under constant LD 12:12. These gonadal cycles were different from those under natural changes in day length in that the period of photorefractoriness, when birds are unresponsive to an increase in day length, was always much less and in some cases very short indeed. The relationship between molt and gonadal regression was similar to that during natural cycles: every bird that showed gonadal regression began to molt during regression, and molt did not occur except during gonadal regression. The plasma-luteinizing hormone showed rhythmicity related to gonadal maturation, although less clearly in females and much less pronounced than during natural annual rhythms. Plasma prolactin remained low in most males but varied erratically in females. There was no evidence of rhythmicity in prolactin in either sex. High prolactin was not related to gonadal regression or molt. The results suggest that if endogenous circannual rhythms have a role in the expression of natural annual cycles in this species, then they are highly modulated by ambient day length throughout the year rather than simply being synchronized by day length.