In many species, embryos are exposed to maternal hormones in utero, in the egg, or in the seed. In birds, mothers deposit substantial testosterone into their eggs, which enhances competitive ability of offspring. These maternal testosterone concentrations vary systematically within clutches in different patterns and may enable mothers to adaptively fine-tune competitive hierarchies within broods. We performed a comparative analysis to investigate this hypothesis using a broad set of avian species. We expected species with small size differences among siblings (arising from small hatching asynchrony or slow growth rates) to aim for survival of the whole brood in good years and therefore compensate last-hatching eggs with relatively more testosterone. We expected species with large size differences among siblings (large hatching asynchrony or fast growth rates) to produce surplus young as insurance against failed offspring and to facilitate elimination of redundant surplus young by bestowing last-hatching eggs ... : Sources for yolk testosterone used in this study.Mean yolk testosterone concentrations in core eggs, marginal eggs and the difference in yolk testosterone between core and marginal eggs. Marginal eggs represent the later-laid, asynchronously-hatching eggs and were identified by dividing the hatching spread by the laying interval between subsequent eggs (app. C). If these calculations produced non-integers, we rounded the values up. Core eggs represent the early-laid, synchronously-hatching eggs and were identified by subtracting the number of marginal eggs from the average clutch size. Yolk testosterone concentrations for core and marginal eggs were averaged to produce mean core T concentrations and mean marginal T concentrations, respectively. In our analyses, we included mean marginal T minus mean core T as a response variable and mean core T as a covariate that corrects for differences in average clutch T values across species as well as assay differences. * indicates the species excluded from analyses ...