Effects of ambient environmental factors on seasonal cycles of the male tree sparrow ( Passer montanus ). Annual changes in temperature, day length, relative humidity and rainfall are presented as means for the months. Birds (n = 7) were capture in the middle of every month and observations were recorded on the changes in testicular volume, bill color, body weight and molt of wing primary and body feathers. The data are presented in means ± s.e.m. Though a slight increase in testicular volume was observed during February to March, a significant testicular growth was noticed only in the month of April with the increasing day lengths in spring. Molting of wing primary and body feathers progressed with gonadal regression showing definite annual cyclicity. No significant changes in body weight were observed throughout the study. The birds posses distinct annual cycles of testicular growth and bill color that showed close correspondence with the annual day length cycle.