Environmental and endocrine control of reproduction in the song sparrow, Melospiza melodia. I. Temporal organization of the breeding cycle.
Endocrinologic investigations of free-living populations of song sparrows, Melospiza melodia, have revealed temporal patterns of secretion of reproductive hormones that differ from those of other monogamous avian species. Males arrive in the breeding area in March whereas females arrive 1-2 weeks later. In males the periods of territory establishment and attraction of a mate are characterized by high circulating levels of luteinizing hormone (LH) and testosterone (T), whereas testis mass is low, and growth of the cloacal protuberance (CPL) is just beginning. In April, testes and CPL develop rapidly reaching a peak in early May when females are laying eggs, and when most copulations occur. Plasma LH and T decline in early April, but increase for a second time in late April and early May coincident with the egg-laying period. Thereafter, circulating LH and T decline during the parental phase, but not to basal levels. Although there is an increase in LH during the egg-laying period for the second clutch, there is no change in T levels. Testis mass, CPL, plasma LH, and T all decline to basal levels simultaneously in late July and August. The high levels of LH and T in March, followed by a temporary decline and resurgence in April and May, indicate that environmental factors in addition to the well known effects of increasing day length can regulate secretion of these hormones. In females, plasma levels of T and dihydrotestosterone (DHT) are also elevated in March and early April, and decline to low levels as the nesting phase progresses. A further decline, to basal levels, occurs in August during the molt. LH and estradiol (E2) titers in females show two peaks coincident with the egg-laying periods for each clutch. Plasma levels of corticosterone (B) increase during the breeding season in males, but not females. In both sexes B levels are basal during the moult and increase in October after moult is completed. Body mass and fat depot decline in males as the nesting phase progresses, and then increase dramatically after breeding is terminated. As expected, females show two peaks of body mass and fat depot coincident with the two periods of egg laying. The postbreeding increases in body mass and fat depot are much less pronounced in females than in males.