written by Tara Haelle

Magic words and passwords are fun – and good learning tools. We teach our children to say the “magic word” when they ask for something, and many of us teach our children a password to use with school pick-ups so they don’t go home with a stranger.

But there’s one bird species that one-ups us humans on special passwords – the superb fairy-wrens of southeastern Australia. Not only do they teach their chicks a special secret password note, but they do it before the chicks even hatch. And just like humans, they’re trying to keep the strangers away. These are the findings of Diane Colombelli-Négrel and her colleagues in a Nov. 8 study in Current Biology.

See, fairy-wrens are a bird species who are sometimes exploited by a “brood parasite.” Brood parasites are animals that use the parenting of a different species to save themselves all that energy of raising their young. It’s like having a baby and dropping it off in your neighbor’s bassinet to deal with to save you the trouble. In the case of superb fairy-wrens, the brood parasite is the Horsfield’s bronze cuckoo. Momma cuckoos lay their eggs in fairy-wren nests with the expectation that once the cuckoo chick hatches, he’ll reap the benefits of having a fairy-wren mother to feed and protect him.

But fairy-wrens have adapted to this evolutionary trick with a clever one of their own. A little more than halfway through their 14-day incubation, starting on day nine, fairy-wren mothers sing an “incubation song” while sitting on their eggs. Every four minutes, she sings a two-second tune, and the little growing chicks in her eggs are listening… and learning.

She sings daily until the eggs hatch five days later, and then the chicks do what any other newborn bird does: they beg for food. But their begging cries contain a single unique note pulled from their mother’s song. That note becomes a password letting the mother fairy-wren know that each of these chicks are really hers.

So what about the cuckoos? Well the mother cuckoos usually drop off their eggs just a few days before the fairy-wrens hatch – too late in the incubation period for the cuckoo chicks inside to learn the password note. When a cuckoo posing as a fairy-wren hatches, he doesn’t know the password of his fake step-brothers and sisters, so he doesn’t incorporate that single special note into his cries. And so the fairy-wren mother ignores him. In fact, when she notices there’s interloper in the nest, she and her mate will usually fly off and make a new nest elsewhere.

Colombelli-Negrel and her colleagues discovered this unique password adaptation through a series of cross-fostering experiments. They observed 15 fairy-wren nests during incubation periods, when they heard the mothers singing to their eggs. When they swapped eggs of fairy-wrens among the nests, the newly hatched chicks begged for food using the special note of the mother who incubated them, not the foster mother whose nest they hatched in. When the researchers played a loudspeaker under a nest with the wrong begging call, the mother fairy-wrens didn’t feed their chicks.

Fairy-wrens stick with their mates for life, so dad is involved in caring for the chicks as well. But often so are other males because even though the fairy-wrens are socially monogamous, they tend to have open marriages – both males and females will mate with others, and a clutch of eggs is often the result of different more than father.

The female fairy-wrens therefore make sure that dad and any other helpers know the secret password by singing them a “solicitation song” away from the nest. If dad or any other helpers are assisting with feeding, then, like mom, they only feed the chicks who sing the secret password.