The Endocrinology of a Natural Breeding Population of the White-Crowned Sparrow (Zonotrichia leucophrys pugetensis)

The levels of immunoreactive luteinizing hormone (irLH), testosterone, dihydrotestosterone (DHT), estradiol, estrone, and corticosterone have been measured through the course of the breeding season in the plasma of both sexes from a breeding population of white-crowned sparrows, Zonotrichia leucophrys pugetensis, on Camano Island, Washington. Birds were captured in nets or traps. After removal of the blood sample and performance of laparotomy to ascertain the developmental stage of the reproductive system, the birds were marked with unique combinations of colored leg bands for identification in field observations and on recapture for procurement of blood samples and for laparotomy. Males arrive on the breeding ground in late April with levels of irLH and testosterone significantly higher than in winter and with partially developed testes. During the ensuing period of establishment of territory, arrival of females, mating, and courtship, the testes attain full development, and irLH and testosterone reach maximum levels in the plasma. Females arrive after territories have been established by the males. Ovaries have begun development at that time; estrone and estradiol are detectable in the plasma; irLH and testosterone are at winter levels. During pairing, courtship, and mating, the ovary develops rapidly, and the plasma levels of irLH, estrone, and testosterone become conspicuously higher. The period of elevated testosterone is brief and coincides with the time during which the females participate in territorial defense. It returns to the winter level before the onset of oviposition at which time estrone has reached maximum level. Regression of the ovary during incubation is coincident with decreases in the levels of irLH and estrone. At this time in the males there is a decrease in irLH and a precipitous decrease in testosterone, which remains low, but above winter level, for the remainder of the year. There is a slight regression in testicular weight. Prior to the laying of the second clutch, the plasma level of irLH increases again but less conspicuously than in females. During the time of the second clutch, there is an increase in plasma androgens in females but not in males. Testes and ovaries return to the level of development attained prior to the first clutch. At the time of oviposition, plasma estrone in females is again maximal. By the time of fledging of the second brood, the levels of irLH and estrone are declining precipitously and gonads are regressing. During the postnuptial molt, testosterone, estrone, and irLH reach winter levels. In females maxima in plasma levels of corticosterone coincide with the two periods of oviposition. The single maximum in males occurs during the time of the second brood. During feeding of the young of the second brood, the corticosterone levels decline in both sexes and reach minimum levels during the postnuptial molt.