Behavioral and hormonal responses of male song sparrows to estradiol-treated females during the non-breeding season.

Previous studies on several species have shown that behavior of female vertebrates given estradiol implants to maintain sexual receptivity resulted in elevated levels of testosterone in males and even delayed termination of breeding in their otherwise untreated mates. However, the efficacy of such treatment in the non-breeding season was unclear. In this study, female song sparrows, Melospiza melodia morphna, given implants of estradiol in autumn showed elevated sexual behavior--especially "chitters," a loud, female-specific vocalization used mostly during the breeding season. Males in autumn appeared to ignore the sexual behavior of these females, showed no difference in response to simulated territorial intrusion, and had basal circulating levels of testosterone when compared with males tested in an area of control females. However, by late winter (February) males in the area where females were treated with estradiol had elevated levels of testosterone compared to control males and those of both groups sampled in autumn. These data indicate that sexual behavior of females, experimentally induced by estradiol implants, failed to increase sexual and territorial behavior in males during the autumn. On the other hand, reproductive behavior of females did appear to accelerate the vernal increase in testosterone concentrations in blood of males.