The Annual Cycle of Plasma irLH and Steroid Hormones in Feral Populations of the White-crowned Sparrow, Zonotrichia leucophrys gambelii

Plasma levels of immunoreactive luteinizing hormone (irLH) and 5 steroid hormones have been measured through the course of the winter breeding season and vernal and autumnal migration in both sexes of the White-crowned Sparrow, Zonotrichia leucophrys gambelii. Birds were captured in mist nets or traps on wintering grounds in Washington and California and on the breeding area in the vicinity of Fairbanks, Alaska. Blood samples were collected from a wing vein as soon after capture as possible. Birds were then individually marked with a unique combination of 1 or 2 colored leg bands for identification in field observations and laparotomy performed to assess the reproductive state. After sampling, all birds were released for subsequent observation and recapture. During autumn and winter, the levels of irLH and sex hormones in the plasma are low in both sexes. In spring (April and May), they begin to increase becoming maximal coincidentally with maximal gonadal weights, establishment of territory, mating and courtship. During incubation and feeding of young, plasma irLH and sex steroid levels decline to basal levels, but there is only a gradual decrease in weight of the testes. By the time parent birds are feeding fledglings, there is a rapid involution of the gonads followed closely by the onset of postnuptial molt. Plasma levels of corticosterone are high in both sexes in January, but much lower in the early spring. In males during vernal migration, there is a dramatic increase to a high level that persists throughout the breeding season. In females, plasma hormone levels remain low during vernal migration, but then increase during the time of ovulation and oviposition. Basal levels in plasma corticosterone occur in both sexes during postnuptial molt. In contrast to vernal migration, a slight increase in plasma corticosterone occurs during autumnal migration in females, but not in males.