RESUMO
Resonance experiment was employed to investigate the mechanism of photoperiodic time measurement during initiation of ovarian growth and functions in the subtropical population of female tree sparrow (Passer montanus) at Shillong (Latitude 25°34´N, Longitude 91°53´E). Photosensitive birds were subjected to various resonance light dark cycles of different durations such as: 12-(6L:6D), 24-(6L:18D), 36-(6L:30D), 48-(6L:42D), 60-(6L:54D) and 72-(6L:66D) h along with a control group under long days (14L:10D) for 35 days. Birds, exposed to long days, exhibited ovarian growth confirming their photosensitivity at the beginning of the experiment. The birds experiencing resonance light/dark cycles of 12, 36 and 60 h responded well while those exposed to24, 48 and 72 h cycles did not. Serum levels of estradiol-17β ran almost parallel to changes in the follicular size. Further, histomorphometric analyses of ovaries of the birds subjected to various resonance light dark cycles revealed distinct correlation with the ovarian growth and the serum levels of estradiol-17β. No significant change in body weight was observed in the birds under any of the light regimes. The results are in agreement with the avian external coincidence model of photoperiodic time measurement and indicate that an endogenous circadian rhythm is involved during the initiation of the gonadal growth and functions in the female tree sparrow.