Doctor of Philosophy
Date of Defense
Robert E. Ricklefs, Ph.D.
Inter-specific variation of the endocrine system is an underlying component for variation in avian life-history. Females deposit hormones into the yolks of their eggs, and embryonic exposure affects nestling phenotype. Hormones are also produced endogenously by developing nestlings, further influencing their behavior and physiology. This dissertation examines the role of the endocrine system in development by focusing on several different avian taxa. Yolk hormone concentrations often vary across the laying order and may mediate hatching asynchrony. These patterns were characterized in two species that lay asynchronously hatching clutches: Eastern screech owls (Megascops asio) and cockatiels (Nymphicus hollandicus). The patterns described in owl clutches could minimize the effects of hatching asynchrony while those in cockatiel clutches could reinforce these effects. These differences may be due to opposing reproductive strategies between the two species. Sibling competition often affects hormone production in nestling birds. While first-hatching screech owls are often more aggressive than younger siblings, androgen concentrations were not related to hatching order. Because androgens can exert physiological costs, owls may minimize production if competition is reduced in captivity. Hormone production in Eastern bluebirds (Sialia sialis) is influenced by sibling competition. Testosterone, but not corticosterone concentrations were higher in nestlings from larger broods. Elevated testosterone may facilitate begging and increase nestling competitive abilities. Nestling hormone concentrations also change with age and mediate important developmental stages. Before fledging, Leach’s storm petrels (Oceanodroma leucorhoa) engage in anorexia to lose excess mass. Hormone concentrations were measured to determine whether the endocrine system initiates this change in feeding behavior. Corticosterone and thyroxine, but not testosterone, increased near fledging. Corticosterone may initiate anorexia while thyroxine may increase nestling metabolism.
Kozlowski, Corinne Patricia, "Avian developmental endocrinology: The effects and role of yolk hormones, sibling dynamics, and life-history" (2010). Dissertations. 456.