Entrainment of oviposition in the fowl using light-dark cycles

1. Fourteen experiments were performed to determine the minimum photoperiod and minimum scotoperiod needed for entrainment of oviposition in hens exposed to 21-h, 24-h and 30-h cycles. Entrainment was measured by the proportion of total eggs laid in a modal 8-h segment of each cycle. 2. In a 24-h light and dark cycle, a 15-min photoperiod or a 5-h scotoperiod produced essentially the same degree of entrainment as 6 h light, 18 h darkness (6L : 18D) or 14L : 10D, which were used as control treatments. Under 21-h light and dark cycles a minimum 3-h photoperiod or a minimum 9-h scotoperiod was needed to achieve full phase setting. When the cycle length was increased to 30 h a minimum 8-h photoperiod or a minimum 12-h scotoperiod was required for full entrainment. 3. This study demonstrates that photoperiod is a stronger signal than scotoperiod for the purpose of phase setting oviposition. It also shows that a stronger signal is required to achieve entrainment when the length of the light-dark cycle is several hours shorter or longer than the natural period of 24 h.