Absence of gonadal-hormone feedback on luteinizing hormone in both photorefractory and photosensitive white-crowned sparrows (Zonotrichia leucophrys gambelii).

Elimination of gonadal-hormone feedback by castration from photorefractory male White-crowned sparrows in mid-July or early October, and from photosensitive males in early December, had no significant effect on plasma levels of LH. Photorefractory birds held on natural daylengths regained photosensitivity in the absence of gonadal-hormone feedback with no increase in plasma levels of LH. Implants of testosterone propionate into castrated males during November and December had no effect on plasma LH levels. These results indicate that gonadal-hormone feedback is not responsible for the very low levels of LH secretion in either the photorefractory or the photosensitive male White-crowned sparrow.

Circadian function in the photoperiodic induction of gonadotropin secretion in the white-crowned sparrow, Zonotrichia leucophrys gambelii.

Photosensitive White-crowned Sparrows (Zonotrichia leucophrys gambelii) previously maintained on short day lengths (8-hr light; 16-hr darkness) were transferred to complete darkness. At various intervals thereafter (2-100 hr) they were exposed to a single 8-hr light period. To determine if the light period had been inductive, plasma luteinizing-hormone concentration was measured by radioimmunoassay in blood samples taken before and after the light period. Inductive light periods recurred at approximately daily intervals. The data provide further evidence for the involvement of a circadian rhythm in time measurement in the photoperiodic responses of birds.