Female Superb fairy-wrens (Malurus cyaneus) call to their eggs during incubation to teach the embryos a vocal element; embryos produce the learned element as a begging call after hatching to solicit high parental feeding. The more a female calls to the embryo, the higher the call similarity after hatching. Female vocal tutors increased incubation call rate to their embryos (and hence copy accuracy in chicks) when brood parasites called near the nest, but high call rate increased predation risk. These findings point to different costs and benefits of teaching effort by female vocal tutors. We predict that female vocal tutors should optimise (rather than maximise) their teaching effort to embryos. We test if female call syntax to embryos predicts the physiological response of embryos during learning tasks and copy accuracy after hatching. Embryos had lower heart rate (higher attention scores) when the call syntax of the female vocal tutor was four repeated elements followed by a novel element (e.g. AAAAB) rather than five repeated elements (e.g. AAAAA). These findings support the idea that female call syntax is under natural selection to enhance prenatal learning under conditions of high predation risk.