Involvement of a circadian rhythm in the photoperiodic ovarian response of the yellow-throated sparrow, Gymnorhis xanthocollis.

Groups of the photosensitive female yellow-throated sparrow were placed under various light-dark cycles, in which a fixed ultrashort photophase of 3 h was combined with dark phases of varying duration, resulting in cycles of 18-30 h. Simultaneously, two groups of birds, one in short days (8L/16D) and the other in long days (15L/9D), were kept as controls. Significant ovarian growth, observed at 30 and 60 days, was induced by 3 h ultrashort photophase only if it was introduced in the cycles of 18 h (3L/15D), 20 h (3L/17D), 26 h (3L/23D), 28 h (3L/25D) and 30 h (3L/27D) as well as under long days (15L/9D), whereas there was no response to the ultrashort photophase in cycles of 22 h (3L/19D) and 24 h (3L/21D) and in short days (8L/16D). It seems that there is an ovarian response to the ultrashort day cycles when a phase advance or delay of photosensitivity of the response system repeatedly produces coincidence of the external photophase (3 h) with the photosensitive phase of an endogenous circadian rhythm. The results are thus consistent with the Bünning hypothesis, which suggests the involvement of an endogenous circadian rhythm in photoperiodic time measurement.