Photoperiodic regulation of reproduction in migratory bunting, Emberiza melanocephala: An evidence for external coincidence model

Abstract

Photosensitive female blackheaded buntings (Emberiza melanocephala) were exposed to various night-interruption cycles for 42 days. These light-dark cycles consisted of a basic photophase of 6 h and 1 h photointerruption of the 18 h dark phase in 24 h cycle at different points. A control group was also placed under short photoperiod (8L : 16D). Ovarian growth and increased plasma estradiol level were evident in night interruption cycle in which photo-interruption of dark photophase was made 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 external coincidence model (Bünning hypothesis), suggesting the involvement of an endogenous circadian rhythm in photo-periodic time measurement in bunting.