Songbirds found using secret code to spot invaders

An Australian scientist has discovered an extraordinary evolutionary trick. Superb fairy-wrens, victimised by parasitic cuckoos, have learnt to defend themselves using secret passwords.

With their bright blue feathers and distinctive song, superb fairy-wrens are – as their name suggests – among the best loved animal inhabitants of southeastern Australia.

But appearances are deceiving. The birds that beautify the sunny gardens of Sydney and Melbourne are locked in a brutal ecological battle. The songs which brighten up each morning are not sung for pleasure: they are evolutionary tools that can make the difference between life and death.

That is particularly true in the case of the superb fairy-wren. A newly published study has shown that the wrens use their song in a way that has never before been seen in the natural world: as a secret password, used to defend against parasitic cuckoos.

Over the millennia, cuckoos have developed a very sophisticated way of getting ahead in the ecological race for survival. They lay their eggs in the nests of other birds. When these cuckoo chicks hatch, they kill their adopted brothers and sisters. Meanwhile, the parent birds – unaware that they are fostering an invader – continue to feed them, spending precious energy to raise a chick that is not their own.

Fairy-wrens are frequent victims of this cruel strategy. But their clever use of song allows them to fight back. A fairy-wren mother, warming her eggs in the days before they hatch, will sing them a song with a special note, unique to each family of wrens, like a sort of sound signature. Even inside the egg, fairy-wren chicks pick up this special note and sing it back to their mother when they want to be fed.

Cuckoo eggs often arrive in host nests later than the host’s own eggs. That means they miss hearing the special song. When they want to be fed, they cannot hit the unique signature note. The parent birds, hearing that something is wrong, can then abandon the young cuckoo and start a new family somewhere else.

This password system is perhaps the most sophisticated anti-cuckoo adaptation ever discovered. But the fairy-wrens’ advantage may not last long. In battles between parasites and hosts, there is rarely an outright victory. Each new weapon on one side is soon matched by an adaptation on the other. So the endless struggle for survival goes on.

Red in tooth and claw

Some animal-lovers might find the fight between cuckoos and fairy-wrens slightly depressing. How sad to think that the natural world is so full of violence and suffering; that the bushes of suburban gardens conceal grisly scenes of avian murder and infanticide.

But biologists are less upset. Indeed, in the story of the cuckoos and the fairy-wrens, most will find cause for wonder at the extraordinary behaviour and adaptation evolution can produce. Nature may be cruel, but it is never boring.

You Decide

  1. What is the world’s greatest bird? Why?
  2. Can an animal ever be evil?


  1. Lots of stories from literature feature talking animals. Write your own, acknowledging the fact that life in the natural world can be bloody and cruel.
  2. Do some further research to find another example of an extraordinary evolutionary adaptation some creature uses to survive. Prepare a short presentation on your example to give to the class.

Some People Say...

“Nature is horrible. The less of it there is, the better.”

What do you think?

Q & A

Glad I’m not a fairy-wren!
Indeed. But we humans are not so far removed from the evolutionary struggle as we like to think.
Oh really?
We have defeated all our obvious animal rivals on the planet. When carnivores like wolves or bears threatened our herds, thousands of years ago, we hunted them close to extinction. Sabre-toothed tigers terrorised our ancestors but it is we who survive today. But we do still have a whole class of deadly enemies.
Everywhere. The biggest threat to the welfare of humanity today may come from bacteria and viruses. They may be invisibly small, but these microorganisms are evolving constantly in an endless effort to overcome our natural defences. One day, they may even succeed.

Word Watch

Superb fairy-wrens
Superb fairy-wrens are small territorial birds in the malurus family. Cousin species include the emperor fairy-wren, the lovely fairy-wren, and even the splendid fairy-wren.
When two species have a relationship in which one side gets all the benefit and the other side suffers, that relationship is called parasitic. The most familiar parasites are things like intestinal worms, which infest the body of a host organism. These are called ‘endoparasites’. Cuckoos, by contrast, are ‘brood parasites’.
Struggle for survival
It is worth noticing, however, that parasites and hosts have different aims in the struggle. The host would like to utterly defeat the parasite. However, if the parasite does too much harm to the host, it will suffer too, as there will be no more victims to exploit. A similar problem affects criminals who run protection rackets, for example. If they extort too much from legitimate businesses, there will soon be no more businesses left to extort.


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