On the recovery of photosensitivity in two passerine species, American tree sparrows (Spizella arborea) and Harris' sparrows (Zonotrichia querula).

To test the hypothesis that a spontaneous increase in plasma luteinizing hormone (LH) signals recovery of photosensitivity in castrated passerine birds, LH concentrations were measured weekly in intact and in castrated photorefractory tree sparrows and Harris' sparrows transferred to short days. After 7 weeks on short days (Experiment 1) or after 1, 3, 5, 7, and 10 weeks (Experiment 2) were challenged with long days (1 week) to determine if photosensitivity had been restored. As evidenced by a significant LH response to photostimulation, tree sparrows had regained at least partial photosensitivity after 7 weeks on short days. However, during exposure to short days, plasma LH concentrations in castrated males did not differ from those in intact males, and plasma LH concentrations in intact or castrated males did not vary with time. The first indication that photosensitivity had been partially restored in Harris' sparrows came after 7 weeks on short days, when castrated males responded to photostimulation with a fourfold elevation in plasma LH concentration. However, before week 7 and through week 10, LH concentrations remained suppressed in both intact and castrated males retained on short days. These data show that recovery of photosensitivity in castrated tree sparrows and Harris' sparrows held on short days is not signaled by a spontaneous elevation in plasma LH concentration, and that plasma LH concentrations in acutely photosensitive intact male tree sparrows and Harris' sparrows held on short days are not suppressed because of gonadal negative feedback.