Smithsonian.com

Photo: David Cook Wildlife Photography

Australian female fairy-wrens don’t even wait until their young are hatched before starting to teach them life skills. The birds sing to their eggs, imparting the embryo inside with a “password”—a unique note that nestlings must later incorporate into their begging calls in order to get food, Nature News reports.

This trick likely evolved as a way to outmaneuver parasitic cuckoos, which frequently infest the fairy-wrens’ nest, often at the cost of the survival of the fairy wrens’ offspring. Though researchers previously knew fairy-wrens could often distinguish their own offspring from invaders based upon calls, no one before understood that the nestlings learned the passwords before emerging from the egg.

The researchers stumbled across the embryonic learning quite by accident. They were recording inside the birds’ domed nests in search of anti-predator calls when they noticed that female fairy-wrens were singing to their unhatched eggs.

To test if the begging call was learned or genetic, Kleindorfer swapped around eggs across 22 nests. When the swapped eggs hatched, nestlings used the call taught by their foster mother, not their biological mother.

Though cuckoo eggs get incubated alongside the legitimate ones, the cuckoo embryos don’t seem to have caught on to the password trick. But the cuckoos have a counter strategy. Some of the young parasites seem to guess the password by trying out random calls until they stumble upon the jackpot and get fed. As usual, adaptations—no matter how extraordinary—only last so long before losing their edge in the evolutionary arms race.

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