Dissertation Research: A Functional and Mechanistic Investigation of Yolk Androgens in the European Starling (Sturnus Vulgaris)

Cornell University, Ithaca NY

Investigators

Abstract

Dissertation Research: A Functional and Mechanistic Investigation of Yolk Androgens in the European Starling (Sturnus Vulgaris) Elizabeth Adkins-Regan and Kevin Pilz Female birds deposit steroid hormones, such as testosterone, in the yolks of their eggs. In captive-bred canaries, high levels of "yolk testosterone" in eggs cause elevated growth rates and begging behavior in the chicks that hatch from those eggs. There is currently no information regarding the behavioral effects of yolk hormones in other species of birds. Nor is there well-substantiated evidence that females deposit different levels of hormones in eggs under different ecological circumstances to maximize their reproductive success. This study intends to fills these gaps in our knowledge by studying two types of "yolk androgens", one being testosterone and the other being a related hormone called androstenedione, in wild-living European starlings (Sturnus vulgaris). The first goal of this research is to document the effects of yolk androgens on European starling chick growth and behavior. Eggs will be injected with hormone or with a control solution and the growth and begging behavior of the chicks will be monitored until the chicks are ready to leave the nest. The second goal of this research is to test the hypothesis that females who engage in the reproductive strategy of brood parasitism deposit high levels of yolk androgens in their eggs. A brood parasite female lays her eggs in the nests of other females to avoid raising the chicks herself; starlings sometimes brood parasitize other starlings. If yolk androgens cause chicks to compete more, by begging more and growing faster, then a female who lays brood parasitic eggs should deposit high levels of androgen in those eggs so that her chicks will be able to selfishly outcompete their unrelated nestmates. The third goal of this research is to examine whether female characteristics such as body size and age are related to the levels of yolk androgens that females deposit in eggs. This data will be used to determine if the small size and young age of brood parasitic females may explain why brood parasitic eggs have high levels of yolk androgens. This data will also be used to test the hypothesis that "high quality" females (larger, older females) should invest more yolk androgens in eggs than "low quality" females. In summary, this research will investigate within a single species of wild songbird, 1) the effects of yolk androgens on chicks; 2) whether females allocate androgens to eggs in ways which benefit their reproductive success; and 3) whether female allocation of yolk androgens varies with female characteristics such as body size and age. This research will increase our knowledge of hormone effects on development in birds and provide understanding of non-genetic means by which females can adaptively manipulate offspring characteristics. The research may also be useful as a model of maternal hormone effects on developing offspring in mammalian species.