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Birds 'Cry Hawk' to Escape Predators

Jun 03, 2015 01:08 PM EDT
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We all know about the boy who cried wolf, but a new study sheds light on brown thornbills, tiny birds that "cry hawk" to escape from predators.

Thornbills weigh only about six grams, and therefore are easy prey for scary pied currawongs, which are 40 times larger. Well, it turns out that brown thornbills, despite their small size, are quite clever creatures. They mimick the hawk alarm calls of neighboring species in order to scare away currawong nest predators that think a bigger, badder predator is on its way - specifically, the brown goshawk.

Normally, currawongs benefit from listening in on hawk alarm calls of other species to zero in on its prey. For them, it's like ringing the dinner bell. However, thornbills exploit this and turn it against them.

As well as issuing their own hawk alarm call, thornbills mimic those of the local species to create the impression of an impending hawk attack. This momentarily distracts the currawong and gives thornbills time to protect their young.

While other animals are known to copy dangerous or toxic species to deter predators, researchers were surprised to see this trick in thornbills.

"The enormous size difference between a tiny thornbill and a 0.5kg goshawk might make it difficult for thornbills to mimic hawk vocalisations accurately, limiting them to mimicking the chorus of hawk alarm calls given by small local species instead," Jessica McLachlan, a PhD student at the University of Cambridge's Department of Zoology, who co-authored the study, said in a news release.

"As hawks are silent when hunting, the alarm calls of local species may be the only sound that warns of a hawk's presence," she added.

To better understand this game of "cry hawk," researchers studied the thornbills and currawongs living in and around the Australian National Botanic Gardens in Canberra. They devised a series of experiments in which they placed stuffed currawongs in front of thornbill nests to test when thornbills use such trickery, followed by experiments testing how currawongs respond to the calls of thornbills.

They found that thornbills used their own and mimicked hawk alarm calls when their nests are under attack. They also found that currawongs delayed attacks for twice as long when mimetic and non-mimetic alarm calls were played together as opposed to non-mimetic calls played alone.

"Distracting a currawong attacking the nest could give older thornbill nestlings a chance to escape and hide in the surrounding vegetation," noted Dr. Branislav Igic from Australian National University, who led the study. "It's perhaps the thornbills best nest defence in this circumstance because physical attacks on the much larger currawong are hopeless."

The findings were published in the journal Proceedings of the Royal Society B.

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