Maternal corticosterone is transferred into the egg yolk

Hormones deposited in the avian egg are considered in many studies to influence or to adjust offspring phenotype to prevailing conditions in an adaptive way. Several studies demonstrated an effect of corticosterone, the main glucocorticoid in birds, injected into the egg on the developing chick, but the injection of steroids in the egg is far from mimicking the natural distribution of the hormone in the egg. Other studies applied a stressor or corticosterone to the mother. However it is still debated whether an increase of circulating corticosterone in the mother translates into higher concentrations of corticosterone in the egg. Therefore, we investigated in captive barn owls Tyto alba whether circulating corticosterone in egg-laying females elevated within a physiological range, resulted in the deposition of corticosterone in eggs. We found that an increase in circulating corticosterone in the mother within the naturally occurring range translated into elevated concentrations of corticosterone in the yolk of subsequently laid eggs, indicating a specific time frame and yolk layer of corticosterone deposition. We conclude that increasing maternal plasma corticosterone within a naturally occurring range is an efficient tool to increase corticosterone concentration in the egg and to manipulate conditions for the developing embryo.