To test the hypothesis that social rearing may induce malleability, socially reared and socially isolated mallard duck, "Anas platyrhynchos," embryos and hatchlings were exposed to the maternal call of a chicken, "Gallus gallus domesticus," until 48 h after hatching. The hatchlings were then tested with the chicken call versus the mallard maternal call at 48 and 65 h. Social rearing before and after hatching led to the development of a persistent preference for the maternal call of the chicken. Social isolates did not develop a preference for the chicken call. The preference for the chicken call in the socially reared birds required both prenatal and postnatal exposure; socially reared birds exposed to the chicken call only before or only after hatching did not develop a preference for it. Social rearing overrode the usual canalizing influence of exposure to the embryo's contact call. Socially reared birds spend most of their time asleep, so there may be a reduction in contact calling and/or less exposure to visual stimulation (less intersensory competition). Malleability is the requisite first step in the behavioural pathway to evolution.