Development of Photorefractoriness in Intact and Castrated Male Starlings (Sturnus vulgaris) Exposed to Different Periods of Long-Day Lengths

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Abstract
The breeding season of many species of birds is terminated by the onset of photorefractoriness; birds become refractory to the stimulatory effects of long-day lengths, and the gonads spontaneously regress. To determine what period of exposure to long-day lengths is necessary to induce photorefractoriness in the European starling (Sturnus vulgaris), intact and castrated males were first maintained on day lengths of 11L:13D per day for 20 wk. This day length is long enough to cause maximum testicular development in intact males and maximum plasma luteinizing hormone (LH) levels in castrates but not long enough to induce photorefractoriness. Both intacts and castrates were then divided into six groups. A control group was kept on 11 L:13D. Other groups were transferred from 11 L: 13D to 18L:6D for periods of 1, 3, 7, 14, or 21 days and then returned to 1 L: 13D. Six weeks after the treatment groups had initially been transferred to 18L:6D, all six groups were again transferred from 1 L:13D to 18L:6D to test whether they were photorefractory. Frequent blood samples were taken and then assayed for LH, prolactin, and thyroxine. Testicular size was regularly measured in intact birds, and molt was recorded in all birds. At least 7 long days were required to induce photorefractoriness and molt. Exposure to I or 3 long days did not induce photorefractoriness or molt but predisposed birds to become photorefractory rapidly when again exposed to long days. In intact birds, LH levels increased immediately after transfer to 18L:6D, but in birds exposed to 14 or 21 long days, it then declined to low levels and did not increase on return to 18L:6D. In castrates, LH also decreased in birds exposed to 14 or 21 long days but remained elevated in other groups. In both intact and castrated birds, at least 7 long days were required for increased prolactin secretion, but levels declined rapidly on return to 11 L: 13D.