Mothers usually set about teaching their offspring the moment they're born. But the females of one Australian bird can't wait that long. 

Superb fairy-wren (Malurus cyaneus) mothers sing to their unhatched eggs to teach the embryo inside a 'password' — a single unique note — which the nestlings must later incorporate into their begging calls if they want to get fed.

The trick allows fairy-wren parents to distinguish between their own offspring and those of the two cuckoo species that frequently invade their nests. The female birds also teach their mates the password.

Fairy-wrens were known to discriminate against cuckoo nestlings on the basis of their foreign begging calls, says Sonia Kleindorfer, an animal behaviourist at Flinders University in Adelaide, who led the work. But it wasn't known that wren nestlings learned the passwords before hatching.

“It has never been shown before that there is actually learning in the embryo stages,” says Kleindorfer. The finding, published today in Current Biology, has the potential to open up new lines of enquiry into prenatal learning in systems other than parasite-host relationships and in other animals — it could occur anywhere where it's a benefit, she adds.

Accidental discovery

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.

When Kleindorfer and her team analysed recordings made over the full nesting cycle, they found that the wren nestlings in a given nest all had the same begging call, which was unique to their nest. That call contained a signature element present in the call the mothers had made while incubating the eggs, and in the call she used to solicit food from the father. When the researchers broadcast a foreign nestling call at the nests, both the female and male adult birds refused to feed the chicks.