One of the tiniest birds in Australia has evolved a unique way of protecting its nest and young from predators by mimicking warning calls of other species, scientists have discovered.

The small brown thornbill, weighing less than a fifth of an ounce, mimics the specific hawk warning call of a number of bird species to scare off predators such as the large pied currawong by convincing it a much large and fiercer predator — the brown goshawk — is approaching.

"It's not superbly accurate mimicry, but it's enough to fool the predator," says Branislav Igic, who studied the thornbill's vocal mimicry as part of his doctoral studies at the Australian National University's Research School of Biology.

Thornbills will use their own hawk alarm calls and mimic the calls of other species to create the impression of an impending hawk attack, distracting currawongs and giving thornbills a chance to protect their young, the researchers report in the Proceedings of the Royal Society B.

"Distracting a currawong attacking the nest could give older thornbill nestlings a chance to escape and hide in the surrounding vegetation," Igic says. "It's perhaps the thornbills' best nest defense in this circumstance because physical attacks on the much larger currawong are hopeless."

The pied currawong can be 40 times the size of a thornbill, so a physical attack would almost certainly fail, he says.

The thornbills' artful deceit was discovered during experiments conducted for another purpose entirely, meant to record birds' reactions to another predator, a stuffed owl, says research group leader Robert McGrath.

"I was puzzled because I could hear the alarm calls of robins, honeyeaters and rosellas, but I couldn't see any," he says.

"I soon realized that the brown thornbill was mimicking the other species, and Branislav later discovered that they sometimes lie about the type of predator present when defending their nests," he added.

Although various forms of vocal mimicry are common among birds, their exact function has been difficult to determine.

The researchers say theirs is the first study to identify a strategy of using vocal mimicry to scare predators.

Using replica thornbill nests stocked with pieces of chicken, the researchers studied the currawongs' reactions to thornbill warning calls.

When they played a recording of thornbills' "trick" calls mimicking other species, the currawongs were distracted for around 16 seconds, which would be enough time for thornbill nestlings to flee or seek cover, the researchers say.

In contrast, the thornbill's own genuine hawk warning calls distracted the currawongs for only half as long, they found.