Thornbills cry wolf in many voices

Nesting birds might not outmuscle many predators, but they can outwit them – as the brown thornbill of southeast Australia shows. The little bird scares off predators by mimicking the hawk warning calls of other birds. It’s the first example of a bird using vocal mimicry to scare predators this way, Australia National University researchers report in the Proceedings of the Royal Society B in June.

Lead author Branislav Igic was conducting fieldwork at the National Botanic Gardens in Canberra when he realised he was hearing a range of alarm calls from birds he couldn’t see. It turned out the tiny brown thornbill was responsible for them all. Naturally, he wanted to figure out why.

It all comes down to creating as convincing a ruse as possible.

Brown thornbill nests are hidden in dense thickets a metre or less off the ground – even so, their young are constantly under threat from much larger crow-like pied currawongs. Currawong chicks devour the chicks of smaller birds – with each brood needing around two kilograms of food. More than a quarter of thornbills lose entire nests to predators.

While many birds use trickery to protect their nests – some feign injury, or even tend to a fake nest, to draw predators away from their brood – the brown thornbill’s approach has never been seen before. When thornbills hear their chicks in distress, they simultaneously emit the alarm calls of up to four different birds, simulating what would happen if a hawk were flying overhead. Tricked into thinking a bigger predator is in the air, the currawongs quickly abandon their hunt.

“The study is very surprising in terms of the complexity of between-species interactions it reveals,” says University of Melbourne ornithologist Michelle Hall. “Birds use vocal mimicry to increase their vocal repertoire, but as far as I know its effect on predator behaviour has not been shown.”

To test if the thornbills were employing this strategy to protect their nests, Igic and his colleagues recorded how nesting thornbills responded to stuffed taxidermy mounts of currawongs and harmless rosella parrots. As expected, the thornbills didn’t react to the rosellas. They didn’t respond to the stuffed currawong on its own either, but mimicked alarm calls when they were presented with the stuffed currawong and sound clips of distressed thornbill chicks, suggesting the trick is geared towards helping their chicks escape an attack.

Sure enough, the currawongs were startled by the alarm calls – regardless of whether they were genuine or mimicked. The researchers played back real honeyeater alarms and honeyeater alarms mimicked by thornbills to currawongs approaching a specially-designed feeder. As soon as the currawongs heard the alarm calls, they either scanned the air for danger or flew away. The researchers think this short distraction buys thornbill nestlings enough time to scatter into the dense vegetation around the nest.

Animals that mimic for self-defence usually pretend to be more dangerous than they are, but this is the first time an animal has been recorded mimicking a fellow harmless species. Instead of pretending to be dangerous, thornbills send false alarms that danger is present. Mimicking predators is out of the question, as predators such as hawks stalk their prey in silence.

Thornbills are talented mimics, reproducing the alarm calls of around 10 species. Why go to the trouble of mimicking so many different alarm calls at once? Igic says it all comes down to creating as convincing a ruse as possible. “You’d be much more convinced if there were five people crying wolf rather than just one,” he says.