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Cuckoos, Wrens in Escalating Evolutionary Arms Race

By James Owen in England
for National Geographic News
March 12, 2003
 
The chick grows into a grub-guzzling monster up to five times the size of its exhausted "parents." Yet they still don't realize their big baby is actually an imposter. Birds that bring up young cuckoos are unable to distinguish between parasitic nestlings and their own.

Until now, that is.

Scientists have discovered there is a bird that can detect cuckoo chicks in the nest. They say this marks the newest line of defense in an ongoing evolutionary "arms race."

Results of the three-year study are published in the March 13 issue of the science journal Nature. It's the first study to show that birds have learned to recognize and reject cuckoo nestlings.

While many birds have become skilled in identifying a cuckoo egg left in their nest, once the egg hatches they are completely duped and raise the chick as their own.

However scientists who monitored populations of the superb fairy-wren in New South Wales, Australia, found that the bird abandoned Horsfield's bronze-cuckoo chicks to starve in 40 percent of cases. The superb fairy-wren is a favorite host species for this cuckoo.

Rebecca Kilner, from Cambridge University in England, was among the scientists who studied the relationship between the two birds.

She said: "While many host birds are very good at spotting cuckoo eggs in nests, we never came across a case where a superb fairy-wren recognized this cuckoo's eggs."

Naomi Langmore, from the Australian National University, who led the study, added: "The cuckoo lays such a good mimetic egg that the wrens have little chance of detecting it in their dark, dome-shaped nests."

The researchers suggest the superb fairy-wren's rejection of Horsfield's bronze-cuckoo nestlings is an evolutionary response to the cuckoo's success in mimicking their eggs. With the first line of defense breached, another is created.

Chicks Home Alone

The study shows there are two pointers that enable wrens to discriminate against parasitic chicks. Suspicion is first aroused if breeding wrens find a nestling home alone, as the imposter will eject all the natural offspring.

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."

Reed Warbler

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 way—and relearn the art of parenting.

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