Ketterson 9701334 Recent investigations have revealed the presence of maternally-derived steroid hormones in the yolks of avian eggs. However, many questions concerning both the functional significance and the mechanism of action of yolk steroids remain unanswered. By concentrating on the interactions between hormones, physiology, and behavior, the proposed research will elucidate the role yolk steroids in the development and survival of offspring in the context of hatching asynchrony. In doing so, the mechanisms by which yolk steroids facilitate growth and survival will be identified. Preliminary research on the eggs of red-winged blackbirds, Agelaius phoeniceus, has shown that the concentration of yolk testosterone varies with laying order in such a way that later-laid eggs contain more hormone. Therefore, this research will focus on the developmental and fitness effects of yolk testosterone by: (1) Establishing whether yolk testosterone levels are correlated with growth rate, muscle development, or begging intensity. (2) Demonstrating causation between yolk testosterone levels and these physiological and behavioral variables by injecting testosterone into yolks. (3) Investigating the effects of female age on yolk testosterone levels in a system where older females produce a greater number of fledglings than first-time breeders.
