The second clue comes from the food-begging call of the chick, which differs slightly from that of real wrens. A reciprocal adaptation to cuckoo parasitism has enabled parents to tell the difference.
But the arms race doesn't end there. "The host bird develops its defenses and then the cuckoo counterattacks," said Kilner.
Having originally focused its energies on copying the wren's eggs, Horsfield's bronze-cuckoos now try to mimic its begging call.
"If we put a different species of cuckoo chick in the wren's nest, one that makes completely the wrong begging call, then it was always abandoned," Kilner said. "Only Horsfield bronze-cuckoo chicks make the right sound to escape detection."
This was reflected in tests that measured the frequencies of begging calls from the Horsfield bronze-cuckoo and the shining bronze-cuckoo, a closely related species, against those of the wren. The sound made by the specialist cuckoo was far more accurate.
But with 40 percent of Horsfield bronze-cuckoo nestlings being detected and left to die, there is still a fair bit of fine-tuning to do.
So why do superb fairy-wrens appear to be unique in their ability to desert cuckoo nestlings? It seems an obvious strategy, so why haven't other birds adopted it?
Langmore says the answer comes down to the costs and benefits of rejecting cuckoo eggs versus chicks.
"Rejecting cuckoo eggs is better than rejecting the chicks," she said. "That's because it saves the current clutch of their own eggs. Cuckoo chicks would otherwise throw out the host's eggs when they hatch."
The egg-rejection strategy is used by birds like the reed warbler in Europe.
"There would be few benefits in reed warblers rejecting cuckoo chicks because in Europe the breeding season is relatively short," Langmore said. "By the time a cuckoo chick had hatched there would be little opportunity left to re-nest.
"By contrast, in Australia the breeding season can stretch for six months or more so there is plenty of time to nest again after deserting a cuckoo chick."
So while superb fairy-wrens would be better off abandoning cuckoos at the egg stage, because this has become so difficult it pays to abandon the cuckoo chick instead.
"If they tried to detect cuckoo eggs there is a high chance that they would reject their own eggs," Langmore said. "So in Australia the benefits of rejecting eggs are lower, and the benefits of rejecting cuckoo chicks are higher."
So will there ever be a winner in the evolutionary arms race between cuckoos and their hosts?
"There certainly can be winners," Langmore said. "A study has revealed that a small warbler [from the Indian Himalayas] is now so good at discriminating eggs on the basis of size that cuckoos no longer parasitize the species."
Cuckoos con other birds into rearing their chicks because it's so much easier than doing it themselves. "As parasites, cuckoos exploit the hard work of other birds," Kilner said.
But if the defense systems of those birds become sophisticated enough, perhaps cuckoos will eventually run out of evolutionary tricks and lose the arms race. If they do, maybe they could evolve another wayand relearn the art of parenting.
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