Effect of daylength on the rate of recovery of photosensitivity in male starlings (Sturnus vulgaris).

Four groups of castrated photorefractory starlings were transferred from a photoperiod of 18 h light/day (18 L) to photoperiods of 6 L, 8 L, 11 L or 12 L. A control group was kept on 18 L. Plasma concentrations of luteinizing hormone (LH) were low in all groups initially. The first significant increase in LH, which signals the recovery of photosensitivity, occurred after 4 weeks in the groups on 6 L and 8 L, after 8 weeks in the 11 L group and after 13 weeks in the 12 L group. There was no increase in the group on 18 L. The rate of recovery of photosensitivity is therefore inversely proportional to daylength, for daylengths between 8 L and 12 L.