Although cuckoo eggs get incubated alongside the wren's eggs, it seems that cuckoo embryos don't have enough time to learn the password well. The lessons begin about 10 days after the eggs are laid, giving wren embryos around 5 days to pick up the call before hatching, but cuckoo embryos, which hatch earlier and then push out any other eggs, only get about 2 days. This means that victimized parents can escape having to feed an enormous baby bird that isn't their own, and can leave to start a new nest.
Wren's aren't perfect at spotting cuckoos, though. They can always identify one species, but catch the other only around 40% of the time. Kleindorfer says there is evidence that, in the latter species, the cuckoo nestlings attempt to guess the password by trying out different calls.
Martin Stevens, a behavioral ecologist at the University of Cambridge, UK, says that the study has implications beyond birds battling cuckoos. “It opens up the possibility that adults could communicate information to their young even before they have hatched,” he says.
Kleindorfer agrees. “There are many different scenarios where mother-to-egg communication would be useful,” she says, “for example to identify relatives or non-relatives.” It could also offer females an extra chance to favor certain cultural traits in the next generation. “It is a new perspective on the battle of the sexes,” says Kleindorfer.