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Acquisition of photosensitivity in castrated male starlings (Sturnus vulgaris) under short day photoperiods

Photorefractory castrated male starlings were transferred from a photoperiodic schedule of 18 h light:6 h dark to 1 h light:23 h dark, 5 h light:19 h dark, 8 h light:16 h dark or 11 h light:13 h dark. Plasma concentrations of LH were measured in blood samples taken at regular intervals, to give an indication of the reacquisition of photosensitivity under these schedules. Concentration of plasma LH increased after 35, 38, 35 and 63 days, respectively; there was a steady increase in LH in all groups, but the increase under 11 h light:13 h dark was much slower than in the other groups. In contrast, birds held under 18 h light:6 h dark showed no increase in LH. Thus, starlings became photosensitive at a similar rate under fixed photoperiods of 8 h light:16 h dark or shorter, but took longer to acquire photosensitivity under 11 h light:13 h dark.