Learning a foreign language is a challenge, but sometimes it's necessary. That's especially true for Australian fairy-wrens, who eavesdrop on the alarm calls of a neighboring bird to escape death by sparrowhawk. A new study suggests that the fairy-wrens recognise these warnings not because the calls sound similar to their own but because they have learned what they mean.
Alarm calls are the first line of defense against predators, especially for small social birds preyed upon by raptors. Some species have elaborate call systems (ScienceNOW, 11 January), while others listen to the alerts of other birds in addition to making their own calls. Eavesdropping is a good backup strategy because birds "don't have to rely on ... seeing the predator themselves" and can spend more time foraging, says biologist Rob Magrath of the Australian National University in Canberra, lead author of the study. But how do birds recognize other species' alarm calls as red alerts?
Magrath and his colleagues turned to fairy-wrens and scrubwrens, two tiny Australian insect-foraging birds both preyed upon by raptors. In the first step of the experiment, the team recorded the alarm calls of scrubwrens threatened by the presence of a fake sparrowhawk. The researchers then played back the recordings to groups of fairy-wrens in two different settings: Canberra, where the fairy-wrens and the scrubwrens live together, and the Macquarie Marshes, where only fairy-wrens dwell. The fairy-wrens in Canberra flew for cover when they heard the scrubwren alarm, but those in the Macquarie Marshes ignored the calls.
The findings suggest that fairy-wrens have learned the meaning of the scrubwren alarm call, says Magrath. Scrubwrens and fairy-wrens have very similar alarm calls, he explains, so if fairy-wrens just recognized the warning because it was familiar, the Macquarie Marshes birds would have fled as well. The researchers also showed that fairy wrens can learn to interpret dramatically different warning calls, such as those of the New Holland honeyeater, another common bird in Canberra. Despite the clear differences in pitch and frequency, the fairy-wrens fled when the researchers played recorded honeyeater red alerts, the team reports online today in the Proceedings of the Royal Society B.
Biologist Andrew Radford, a specialist in vocal communication in birds at the University of Bristol in the United Kingdom, agrees that the findings are compelling. "This paper provides the strongest circumstantial evidence to date that learning may indeed play a role" in how birds interpret other species' alarm calls, he says.