By Ed Yong (Discover) 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.
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 have a way of telling their chicks apart from cuckoos. Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.