Effects of exogenous 17 beta-estradiol on tee reproductive physiology and reproductive performance of European starlings (Sturnus vulgaris)

Egg mass shows large intraspecific variation in birds and yet the mechanism s underlying this variation remain unknown. We hypothesized that estradiol would play a central role in determining egg mass, since this hormone stimu lates the production of yolk precursors (vitellogenin and very-low density lipoprotein, VLDL) by the liver, and of albumen by the oviduct. We gave Eur opean starlings (Sturnus vulgaris) silastic implants containing estradiol p rior to egg formation, which we predicted would increase egg mass. As expec ted, exogenous estradiol stimulated a marked (49 %) increase in plasma vite llogenin levels at the beginning of laying. At clutch completion, plasma VL DL levels and oviduct mass were also elevated in estradiol-treated females compared with controls. However, estradiol had no effect on fresh egg mass or clutch size. Estradiol treatment actually decreased the mass of yolk pro tein and lipid, perhaps by decreasing the rate of uptake of yolk precursors at the ovary. The failure of estradiol to increase egg mass indicates that this phenotype may be regulated at higher levels of organization (e.g. neg ative feedback, uptake of yolk precursors) than those studied in this exper iment. Despite elevating yolk precursor levels, treatment with estradiol ha d no effect on the mass of the liver or endogenous stores of protein and li pid at clutch completion.