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In Australia, a pair of superb fairy-wrens return to their nest with food for their newborn chick. As they arrive, the chick makes its begging call. It’s hard to see in the darkness of the domed nest, but the parents know that something isn’t right. Whatever’s in their nest, it’s not their chick. It doesn’t’ know the secret password. They abandon it, flying off to start a new nest and a new family somewhere else.

It was a good call. The bird in their nest was a Horsfield’s bronze-cuckoo. These birds are “brood parasites” – they lay their eggs in those of other birds, passing on their parenting duties to some unwitting surrogates. The bronze-cuckoo egg looks very much like a fairy-wren egg, although it tends to hatch earlier. The cuckoo chick then ejects its foster siblings from the nest, so it can monopolise its foster parents’ attention.

But fairy-wrens have a way of telling their chicks apart from cuckoos. Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.

This is one of many incredible adaptations in the long-running battle between birds and their brood parasite. As these evolutionary arms races continue, the parasites typically become ever better mimics, and the hosts typically become ever more discerning parents.

This battle usually plays out before the eggs hatch. If the parents can recognise the parasitic eggs, they’ll eject or destroy them. If they can’t, they often end up feeding the parasite regardless of what it looks like. This is why the common cuckoo has an egg that closely matches that of a reed warbler, but the cuckoo chick is a huge, grey monster that looks completely unlike a warbler chick.

But the fairy-wrens are different. In 2003, Naomi Langmore found that they will abandon 40 percent of nests that only have a Horsfield bronze-cuckoo chick in it, suggesting that they can indeed recognise these interlopers. Now, Colombelli-Negrel has discovered how they do it.

She kept 15 nests under constant audio surveillance, and discovered that fairy-wrens call to their unhatched chicks, using a two-second trill with 19 separate elements to it. They call once every four minutes while sitting on their eggs, starting on the 9th day of incubation and carrying on for a week until the eggs hatch.

When Colombelli-Negrel recorded the chicks after they hatched, she heard that their begging call included a single unique note lifted from mum’s incubation call. This note varies a lot between different fairy-wren broods. It’s their version of a surname, a signature of identity that unites a family. The females even teach these calls to their partners, by using them in their own begging calls when the males return to the nest with food.

These signature calls aren’t innate. The chicks’ calls more precisely matched those of their mother if she sang more frequently while she was incubating. And when Colombelli-Negrel swapped some eggs between different clutches, she found that the chicks made signature calls that matches those of their foster parents rather than those of their biological ones. It’s something they learn while still in their eggs.

This also explains why bronze-cuckoos don’t make the same calls. The female bronze-cuckoo tends to deposit her eggs while a fairy-wren’s clutch is around 12 days old. At this point, they’re just a couple of days away from hatching. “The cuckoo embryo appears to have insufficient time to correctly learn the password note,” says Sonia Kleindorfer, who led the study.

When they hatch, the fairy-wrens can tell that something isn’t right. They spend less time feeding their alien chick, more time making alarm calls, and more time scanning the surrounding area, possibly on the lookout for more cuckoos. In many cases, they abandon the intruder to make a fresh start elsewhere.

This isn’t a clear win for the fairy-wrens. Colombelli-Negrel found very low levels of cuckoo parasitism in her study, but Langmore previously showed that only 40 percent of the wrens abandon their cuckoo-infested nests. “There appear to be cyclical fluctuations in cuckoo prevalance in host nests across years,” says Kleindorfer. This might be because the local wrens become better or worse at recognising their signature notes, or the cuckoos become better or worse at mimicking them. “Our study provides a testable framework to explain some of this variation.”

Reference: Colombelli-Negrel, Hauber, Robertson, Sulloway, Hoi, Griggio & Kleindorfer. 2012. Embryonic Learning of Vocal Passwords in Superb Fairy-Wrens Reveals Intruder Cuckoo Nestlings. Current Biology (2012), http://dx.doi.org/10.1016/j.cub.2012.09.025

Not Exactly Rocket Science
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