Distribution of oviposition times of hens in continuous darkness or continuous illumination.

The pattern of oviposition in continuous illumination or continuous darkness was investigated. In continuous darkness the mean time of all ovipositions was at 05.00 h and the mean time of the first oviposition of a sequence (C1) was 01.00 h. There was considerable variation and some ovipositions occurred throughout the 24 h. In contrast, the mean time of all ovipositions under continuous illumination was at 15.00 h and the mean time of C1 ovipositions was 12.30 h. When a diurnal rhythm of bird noise was audible during the light period (photoperiod 06.00 to 18.00 h or 18.00 to 06.00 h) in adjoining rooms, the birds in continuous illumination showed a mean time of oviposition at 09.30 h with mean time of C1 ovipositions at 06.30 h. For birds in continuous dark the corresponding mean values for the two treatments were 04.00 and 03.00 h for all ovipositions and 00.30 and 23.00 h for the first oviposition of a sequence. When continuous illumination was changed to continuous darkness and vice versa and the 12L:12D lighting treatments in the adjoining rooms were correspondingly reversed, the mean times of oviposition under continuous darkness occurred at 05.00 and 06.30 h, which was earlier by 3 and 4.5 h than the mean time of oviposition at 09.30 h under continuous lighting. The tendency of birds kept in constant darkness to lay eggs in the early morning hours was found to be persistent and repeatable. It is concluded that the findings are best explained by the entrainment of the circadian rhythm to an unidentified synchronising Zeitgeber, but only in continuous darkness.