Evidence against a period of relative photorefractoriness during the recovery of photosensitivity in common starlings.

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.