DECREASED LIGHT-INTENSITY ALTERS THE PERCEPTION OF DAY LENGTH BY MALEEUROPEAN STARLINGS (STURNUS-VULGARIS)

The breeding season of wild starlings is controlled by photoperiod. Fu ll breeding condition is attained during exposure to lengthening days in the spring, and photorefractoriness ensues. The reproductive system of starlings will not subsequently be stimulated by long day lengths until photorefractoriness is dissipated by the short day lengths exper ienced in the autumn and winter Unlike most studies on avian photoperi odism, this investigation involved manipulation of light intensity of a fixed photoperiod rather than of photoperiod itself. Photosensitive starlings transferred from short days to long days of different light intensities underwent graded reproductive responses according to the l ight intensity they experienced. Testes size in the group In the lowes t intensity (3 lux) increased faster than that in controls on short da ys of normal intensity, but they did not become photorefractory. Teste s size increased in the groups on 13, 45, and 108 lux and subsequently became photorefractory. However, the 13- and 45-lux groups required m ore time to become photorefractory than did the 108-lux group. The res ponses observed were similar to those seen in starlings exposed to dif ferent photoperiods (e.g., 11 h light:13 h dark [11L:13D], 13L:11D, 16 L:8D, 18L:6D), even though all were on the same 18L:6D photoperiod. In itially, the results appear to challenge the external coincidence mode l for photoperiodic time measurement, but consideration of the phase r esponse curve of the circadian rhythm of photoinducibility in starling s and the way in which it might be affected by low light intensities r efute this challenge.