Elevated yolk androgen levels benefit offspring development in a between-clutch context

The field of androgen deposition in avian eggs and its consequencesfor offspring development has received a lot of attention inrecent research. However, although variation within clutchesin yolk androgens is relatively well understood, the adaptivesignificance of patterns of variation between clutches remainsrather unclear. Furthermore, it has been hypothesized that yolkandrogens act as a means of an adaptive maternal effect to adjustoffspring to a given posthatching environment. Thus, the consequencesof maternal yolk androgens for offspring development are likelyto depend on the specific environment of a given brood. We experimentallymanipulated yolk androgen concentrations in spotless starlingeggs, using a between-brood design in which full broods weremanipulated applying either an androgen or sham treatment, inorder to test the effects of between-brood variation in yolkandrogen levels. We also included in the analysis several femalecharacters that have been shown to affect androgen deposition.Androgen-chicks tended to gain more mass, a similar effect tothat shown in previous studies where direct competition betweenchicks belonging to different treatments was allowed, but didnot gain a survival benefit. Androgen-chicks had wider beakflanges, an effect that has not been described previously andcould play an important role in food acquisition. In addition,androgen-chicks had higher endogenous plasma levels of androgens,which could induce higher begging intensity. We hypothesizethat these effects are an important mechanistic link for ourunderstanding of how yolk androgens exert their effects on offspringdevelopment after hatching. Contrary to our predictions, wefound no evidence that the observed effects depended on theenvironment under which it was investigated. Because high yolkandrogen levels seem to be beneficial for nestlings in thisspecies, we hypothesize that yolk hormone deposition could becostly for females or alternatively that potential negativeeffects later in adulthood may constrain maternal hormone allocation.