Circadian Rhythmicity and the Termination of Photorefractoriness in the Black-Headed Bunting

Groups of photorefractory male Black-headed Buntings (Emberiza melanocephala), previously maintained on long daylengths (15L:9D), were exposed to photoperiods in which a 6-h light period was coupled with dark periods of 6, 18, 30, 42, or 54 h; a sixth control group was kept on 15L:9D. Cycles with lengths of 24 (6L:18D) and 48 h (6L:42D) terminated photorefractoriness, but cycles of 12 (6L:6D), 36 (6L:30D), and 60 h (6L:54D) did not; refractoriness persisted in the birds on 15L:9D. The results demonstrate that the termination and maintenance of the refractory period in Black-headed Buntings, a palearctic migratory passerine, are mediated by an endogenous circadian system of photoperiodic time measurement similar to that involved in the initiation and maintenance of gonadal growth, as is the case for several nearctic migratory species.