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  • Title: Evidence against a period of relative photorefractoriness during the recovery of photosensitivity in common starlings.
    Author: Dawson A.
    Journal: Gen Comp Endocrinol; 2004 Mar; 136(1):117-21. PubMed ID: 14980802.
    Abstract:
    In many species of birds, the end of the breeding season is caused by the development of absolute photorefractoriness, characterised by spontaneous gonadal regression under a long photoperiod followed by a period during which gonadal maturation cannot be stimulated by further photostimulation. It has been suggested that during the dissipation of photorefractoriness under short photoperiods, birds become relatively photorefractory-that there is a gradual decrease in critical photoperiod. If true, birds should begin to respond to a very long photoperiod before they respond to a moderately long photoperiod. To test this, I transferred photorefractory castrated starlings (Sturnus vulgaris) from a long photoperiod of 18 h of light per day (18L:6D) to 8L:16D for 10, 20, or 30 days, before transferring them to either 20L:4D or to 13L:11D. There was a gradual increase in response (increase in plasma LH concentration) the longer birds had been on 8L:16D before transfer to the longer photoperiods, but there was no difference in the timing of these responses between birds moved to 20L:4D or to 13L:11D. Therefore, starlings do not pass through a period of relative photorefractoriness during the dissipation of absolute photorefractoriness.
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