Seasonal changes in the reproductive functions of Java Sparrows (Padda oryzivora)

1. 1. Birds were paired, a male and female per cage, under a changing light regime which mimicked the changing natural daylength. The temperature was kept constant and feed was provided ad lib. 2. 2. Plasma and tissue samples were collected in March, May, July, September, November and January. The weight of reproductive organs, levels of steroid hormones in plasma and reproductive organs and luteinizing hormone (LH) contents in the pituitary gland were measured. 3. 3. In the male, testicular weight and testosterone levels in plasma and testes were high in March, declined to low levels in May and remained at these low levels in July. All variables started to increase in September. The testicular weights and plasma testosterone concentrations remained high until January whereas testicular testosterone contents declined in January. LH contents in the anterior pituitary were low between March and September but high in November and January. 4. 4. In the female, weights of ovary and oviduct and estradiol concentrations in plasma remained low between March and September, started to increase in November and were maintained at high levels until January. The changes in ovarian estradiol content were similar to the changes in plasma estradiol concentrations. LH contents in the anterior pituitary decreased in July increased from September and remained high until January. The results suggest that the reproductive activity in Java sparrows may be stimulated by a decrease in daylength at constant environmental temperature with food available ad lib.