The decline in yolk progesterone concentrations during incubation is dependent on embryonic development in the European starling

► We examined the metabolism of exogenous progesterone in European starling eggs. ► We also describe a focal freezing technique that terminates embryonic development. ► Embryonic development is critical for in ovo metabolism of progesterone to occur. ► Levels of water-soluble metabolites increase over the first 5 days of incubation. ► Progesterone is converted to a polar metabolite that is subsequently conjugated.