Birds cry wolf to scare predators

Researchers from The Australian National University (ANU) found that the tiny brown thornbill mimics the hawk warning call of a variety of birds to scare off predators threatening its nest, such as the larger pied currawong.

"It's not superbly accurate mimicry, but it's enough to fool the predator," said Dr Branislav Igic, who carried out the study during his PhD at ANU Research School of Biology.

"A physical attack on a currawong would be no good. They are 40 times the size of a thornbill and will eat adults as well as nestlings.

"I am amazed that such a tiny bird can mimic so many species, some much bigger than itself. It's very cunning," said Dr Igic, who has now taken up a position at the University of Akron, in Ohio, United States.

Although vocal mimicry is widespread amongst birds, its function is rarely understood. This study is the first to show that birds use vocal mimicry to scare predators.

The researchers stumbled across the thornbill's deceit during an experiment on birds' reaction to a stuffed owl, said Professor Robert Magrath, the leader of the research group at the Research School of Biology.

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

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

The researchers also used fake thornbill nests populated with chicken pieces to study the reaction of currawongs, which will typically feed a single brood about two kilograms of baby birds in a season.

When the researchers played recordings of the thornbill's trick calls, the currawongs were distracted for around sixteen seconds, which would be enough for nestlings to flee or seek cover.

The thornbill's own hawk warning call distracted the currawongs for only half as long.

The deception succeeds because currawongs eavesdrop on other species, Professor Magrath said.

"Many species of both birds and mammals eavesdrop on the alarm calls of other species. Natural communities form an information web about danger.

"Currawongs would normally benefit, because they are also vulnerable to hawks, but thornbills turn this against them," he said.