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.

Continued on Next Page >>


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