PHOTOPERIODIC REGULATION OF REPRODUCTION IN MIGRATORY BUNTING, EMBERIZA-MELANOCEPHALA - AN EVIDENCE FOR EXTERNAL COINCIDENCE MODEL

Photosensitive female blackheaded buntings (Emberiza melanocephala) we re exposed to various night-interruption cycles for 42 days. These lig ht-dark cycles consisted of a basic photophase of 6 h and 1 h photoint erruption of the 18 h dark phase in 24 h cycle at different points. A control group was also placed under short photoperiod (8L:16D). Ovaria n growth and increased plasma estradiol level were evident in night in terruption cycle in which photointerruption of dark photophase was mad e 13 h and 15 h after the onset of basic photophase, i,e. 6L:7D:1L:10D and 6L:9D:1L:8D cycles. The present results are consistent with exter nal coincidence model (Bunning hypothesis), suggesting the involvement of an endogenous circadian rhythm in photoperiodic time measurement i n bunting.