Cuckoo Chicks Change Calls to Mimic Host

The chicks of a species of Australian cuckoo can adjust their call in order to fool other species into rearing them, despite never having heard the cry, researchers have found.

Like their European counterparts, Australian cuckoos are well known for laying their eggs in the nests of other birds.

Once the chicks hatch, they kick out the host's other eggs and set about convincing their foster parents to feed them by imitating the calls of the host's offspring.

But researchers from the Australian National University and the University of Cambridge, report in the latest issue of the journal Evolution, that one species of cuckoo can modify its call depending on which species it has hooked up with.

Females of the Horsfield's bronze-cuckoo (Chalcites basalis), usually lay their eggs in the nests of fairy-wrens, but will sometimes lay them in the nests of other species including thornbills and robins.

Chicks that hatch in a fairy-wren nest are known to copy that species' short "cheep cheep" begging call, while chicks that hatch in the nests of thornbills imitate the thornbill's long, rasping whine.

Naomi Langmore and colleagues wanted to know how the chicks "decide" which cry to make.

"The most logical assumption was that there would be two races of cuckoo, each specializing on a different host and making a begging call that matches its host," Langmore said. "This would be similar to the European cuckoo, which has several different races each of which lays an egg that matches that of its favored host."

To test whether this was the case, the researchers took cuckoo eggs laid in fairy-wren nests, and switched them into thornbill nests. They then used tiny microphones clipped on nearby foliage to follow what happened.

"We were amazed to find that the chicks could modify their calls," Langmore said.

The chicks first begged like a fairy-wren chick, but within several days had switched to match the length of the thornbill's call.

"Remarkably, they make the same begging call as the chicks of whichever host rears them, even though they never actually hear the host chicks," said Langmore.

The surprising result suggests that the cuckoos could have a range of call options genetically pre-programmed, she said.

Chicks reared by a host other than a fairy-wren might find that they aren't getting fed properly because they aren't making the right call. That could prompt them to apply a simple rule that says "switch to an alternative begging call if I'm going hungry."

"Cuckoos survive by fooling other birds into rearing their chicks, so they are grand masters of deception," said Langmore.

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