Female superb fairy wrens (Malurus cyaneus) in Australia sing to their eggs for a quite specific reason, to teach the embryos a single unique note that is a “password.” The hatchlings must incorporate this special password into their begging calls in order to get fed.
The reason for this prenatal instruction? So the wrens can identify their own offspring from those of two cuckoo species who are considered “brood parasites” because they routinely lay their eggs in other birds’ — in the wrens’ — nest. As researchers write, the “cost of making a recognition error is high” for these avian parents.
Scientists have known that fairy wrens can tell their own nestlings from those of the cuckoos based on their begging calls. But they didn’t know that the wrens teach their yet-unhatched eggs a special, parent-specific password, as Sonia Kleindorfer, an animal behaviorist at Flinders University in Adelaide, says in Nature News. “It has never been shown before that there is actually learning in the embryo stages,” she notes.
The passwords are kept “in the species,” with the female wrens also teaching it to male birds. (You can hear a sample of the incubation call via Nature News.)
The scientists just happened to notice that the wrens were singing to their eggs while recording inside their nests to study anti-predator calls; they noticed that the females were singing to their eggs. By studying the hatchlings’ calls over an entire nesting cycle, the scientists found that there is a password unique to each nest that remains the same for the entire nesting cycle.
Even more, by swapping around the eggs of 22 nests and noting that the hatchlings then learned their foster mothers’ calls, scientists found out that the calls are learned and not genetic. The more times the embryos hear the special calls, the better [their] mimicry,” says one of the study’s authors, Mark Hauber, an animal behaviorist at Hunter College at the City University of New York, in the New York Times.
The female fairy wrens are consistently able to identify one species of cuckoo but only the other only about 40 percent of the time. The cuckoo hatchlings simply don’t have enough time to learn the special calls:
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.
When a cuckoo hatchling is born, it generally throws out the unhatched eggs in the nest. But if the mother wren does not hear the special password — and the cuckoo hatchlings do try to guess it by attempting different calls – she will abandon the nest and start another.
What is intriguing about the study is that it “opens up the possibility that adults could communicate information to their young even before they have hatched,” as Martin Stevens, a behavioral ecologist at the University of Cambridge notes. How many of you who are parents, and mothers in particular, sang to your yet unborn baby in your womb and then were quite convinced that your child, after he or she was born, perked up on hearing those familiar sounds again?
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