Fairy wrens teach secret passwords to their unborn chicks to tell them apart from cuckoo impostors

In Australia, a pair of superb fairy-wrens return to their nest with food for their newborn chick. As they arrive, the chick makes its begging call. It’s hard to see in the darkness of the domed nest, but the parents know that something isn’t right. Whatever’s in their nest, it’s not their chick. It doesn’t’ know the secret password. They abandon it, flying off to start a new nest and a new family somewhere else. It was a good call. The bird in their nest was a Horsfield’s bronze-cuckoo. These birds are “brood parasites” – they lay their eggs in those of other birds, passing on their parenting duties to some unwitting surrogates. The bronze-cuckoo egg looks very much like a fairy-wren egg, although it tends to hatch earlier. The cuckoo chick then ejects its foster siblings from the nest, so it can monopolise its foster parents’ attention. But fairy-wrens ...