The eggs of the canary (Serinus canaria) contain variable doses of maternal testosterone. The reported experiments investigated whether testosterone influences nestling growth and how this interacts with differences of the growth of nest mates that are caused by asynchronous hatching. Injections of testosterone into the yolk of unincubated eggs enhanced the growth after hatching compared to nestlings that had hatched simultaneously from control eggs. These differences were established within 22 hr of hatching. Exogenous testosterone promoted growth in both sexes and there was no sexual difference in the growth of control birds. Testoster-one-treated chicks also begged more often for food. Previous studies have shown that the content of maternal testosterone increases in each subsequently laid egg in a clutch. Consistent with the results obtained by testosterone injections nestlings that hatched from eggs with higher concentrations of maternal testosterone grew faster compared to chicks that hatched synchronously from eggs with lower testosterone concentrations. However, more testosterone did not compensate for reduced growth that was caused by later hatching due to asynchronous incubation of clutches. This direct effect of maternal testosterone on growth in combination with a flexible onset of incubation allows to selectively enhance the growth and fitness of individual offspring of a brood.