Yolk androgens vary inversely to maternal androgens in Eastern Bluebirds: an experimental study

1. Female birds deposit variable amounts of androgens in the yolks of their eggs, and it has been suggested that yolk androgen deposition is an adaptive mechanism preparing offspring for a competitive environment. Breeding pairs of Eastern Bluebirds ( Sialia sialis ) Linnaeus were stimulated with an intruder presentation while ovarian follicles were developing. Yolk steroid concentrations in eggs laid by stimulated females were then compared with yolk steroid concentrations in eggs laid by control females. Additionally, blood samples taken from a subset of control and stimulated females were analysed for plasma steroid hormone concentrations. We predicted that female bluebirds experiencing a simulated intrusion would experience increased levels of circulating plasma androgens that would be reflected by larger amounts of androgens deposited in their eggs compared with control females. 2. Patterns of steroid concentrations differed between egg yolks and female plasma. In egg yolks, androstenedione was the predominant hormone, followed by testosterone. Yolks contained minimal amounts of both corticosterone and oestradiol. In female plasma, however, corticosterone was the predominant hormone, while sex steroids were found at low levels. 3. Yolk steroid concentrations did not vary with laying order in either the control or the stimulated group, a result expected due to the relatively synchronous nature of incubation behaviour exhibited by Eastern Bluebird females. 4. Yolk androgen concentrations in eggs laid by stimulated females were significantly higher than in those laid by control females, suggesting that females increase yolk androgen deposition in response to aggressive encounters. 5. Females exposed to an intruder presentation contained significantly lower levels of plasma androgens than control females. We suggest that the deposition of androgens in the eggs serves as an adaptive method of regulating circulating androgen levels in the female, preventing potentially disruptive elevations in circulating androgen concentrations during a particularly sensitive period in the reproductive cycle.