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

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 ...