Photorefractoriness and its termination in the subtropical house sparrow, Passer domesticus: involvement of circadian rhythm.

Groups of photorefractory female subtropical house sparrows, Passer domesticus, when treated with 6 weeks of a short photocycle (8L:16D) showed significant ovarian growth on their return to a long photocycle (15L:9D). A 6-hr photophase coupled with scotophase of varying durations does not terminate the refractory period under photoperiod cycles of 12 (6L:6D), 36 (6L:30D) and 60 (6L:54D) hr but the refractory period is terminated by light-dark cycles of 24 (6L:18D), 48 (6L:42D) and 72 (6L:66D) hr. These results are consistent with the Bünning hypothesis of coincidence between endogenous photosensitive rhythmicity and environmental photoperiod timing that an endogenous circadian rhythm is involved in the maintenance and termination of photorefractoriness.