Tricksy thornbills cry wolf to scare off predators

Nesting is a risky business. If you're a small bird like a brown thornbill (Acanthiza pusilla) your best defence against predators is probably to fly away. But a nest is a sitting target, so eggs or chicks are an easy meal if a predator finds them. In response to predators near their nests, many birds make alarm calls. These special calls could inform a predator that it has been seen and can't ambush the birds or might attract other small birds to chase away the predator. But could they also be used to fool a predator into thinking there's another threat nearby?That's exactly what researchers from Australian National University set out to test. Lauren Ascah and colleagues studied brown thornbills – tiny birds with big voices – in and around the Australian National Botanic Gardens in Canberra. In response to danger, thornbills often make ‘mobbing alarm calls’ – the sort of alarm calls birds give in response to predators that are grounded such as domestic cats or predatory birds, including currawongs, which perch near their nests ready to eat their eggs or chicks. But when they spot a flying predator, such as a hunting hawk, thornbills make a second type of call, known as an ‘aerial alarm call’. Weirdly, though, when there's a currawong near their nest, thornbills don't only produce mobbing alarm calls, they sometimes make aerial alarm calls too, as if there is a hawk in the vicinity. Sometimes they even mimic the mobbing and aerial alarm calls of other birds, such as New Holland honeyeaters. But why would thornbills act as though there's a hawk or other flying predator nearby, when there's actually a currawong perched near their nest?To test why thornbills make aerial alarm calls in response to nearby currawongs, the researchers attracted 20 currawongs to artificial nests baited with chicken meat, and watched the birds’ reactions when played recordings of various thornbill alarm calls. The currawongs generally responded to the recordings by scanning around for danger and stopping eating. They did this when they heard a combination of thornbills making their own mobbing alarm calls and mimicking the mobbing alarm calls of honeyeaters. However, when the researchers played thornbills’ aerial alarm calls followed directly by thornbills mimicking either the mobbing or aerial alarm calls of honeyeaters, the currawongs reacted much more strongly; they were more likely to look around for danger and stopped eating the chicken meat for longer. Moreover, although the currawongs never flew off when they heard mobbing alarm calls alone, they often fled in response to the thornbill and honeyeater aerial alarm calls. This suggests that the thornbill aerial alarm call, alongside mimicry of other species’ aerial alarm calls, successfully chases off fearsome nest predators.What makes aerial alarm calls so effective against currawongs? Well, mobbing alarm calls are the typical response to a nest predator like a currawong, so currawongs can probably safely ignore those after a quick look around to check that there isn't any other danger. However, an aerial alarm call indicates the presence of an airborne threat like a hawk, so it makes sense for currawongs to flee from this possible danger. In other words, thornbills are crying wolf: tricking currawongs into thinking there's an even scarier predator on the hunt. And mimicking alarm calls of other species might help thornbills convince currawongs that there really is a threat to flee from – since other birds seem to have spotted the fictitious hawk too. And so these devious strategies help tricksy thornbills protect their chicks, who will hopefully live to fight – and eventually scare off currawongs – another day.