There is growing empirical support for teaching in nonhuman animals. To unravel the evolutionary dynamics of teaching, we need to understand its costs and benefits. In superb fairy-wrens (Malurus cyaneus), females teach their embryos by calling to them: embryos learn a vocal password, and hatchlings incorporate the learned vocal password into their begging calls to solicit parental feeding. The more a female teaches (higher call rate), the better the embryos learn (greater call similarity), leading to more food provisioning by parents. Given these direct benefits, we would expect all female fairy-wrens to call often to maximize embryonic learning in their genetic progeny, yet they do not. Teaching the password carries a severe cost: nest predation was higher at both natural and artificial nests that had more incubation calls. At artificial nests, predation was 8-fold higher for high incubation call rate (30 calls per hour) and 5-fold higher for low incubation call rate (15 calls per hour) compared with nests without any incubation calls. At natural nests, nests that were depredated during incubation had higher incubation call rate (18.3 calls per hour) than nests that survived (11.4 calls per hour). Mother fairy-wrens must trade-off the costs of calling and the benefits of learning to optimize fitness benefits of teaching.