More than just twitter
Autotuning avian alarm calls
For a place considered tranquil and quiet, the Australian National Botanic Gardens in Canberra are awash with sound. From the songs of numerous birds to the buzz of leaf blowers, the voices of tour guides to the rustle of wind in the trees. But how does one make sense of this aural assault and detect when predators approach? In the case of the superb fairy wren, the more sets of eyes watching for danger, the better – even if that means listening in on the conversations of your neighbours.
“I study how fairy wrens eavesdrop on the alarm calls of other species,” explains Pamela Fallow, a PhD student in the Research School of Biology at the Australian National University. “They’re a very interesting species, everybody loves them. I’ve probably looked at them a million times and they are still cute.”
Listening to neighbourhood gossip can be of great benefit to fairy wrens, who take advantage of the extra pairs of eyes of their neighbours to get more information about predators. “It’s not just whether danger is present, they can also gain information about which direction the threat is coming from, whether it’s an aerial or ground threat, and how close it is,” Pamela says.
In particular, Pamela is interested in what it is about aerial alarm calls that fairy wrens find so alarming. Do they learn to respond to the calls or is there something about these sounds that they interpret to mean danger?
“We had a good idea which acoustic properties the fairy wrens were likely to be using to detect an aerial alarm call from a previous study - peak frequency, which is the frequency of the call at the point of greatest amplitude, and frequency modulation rate, which is how rapidly the frequency changes over the duration of the call.“
Using audio software, Pamela created synthetic bird calls with different peak frequencies and frequency modulation rates. The response of fairy wrens to these synthetic calls revealed they use these acoustic properties to identify alarm calls.
“It’s a two stage response,” Pamela explains. “Fairy wrens seem to use peak frequency to decide whether to flee or not. Modulation rate was more of an afterthought. If both the peak frequency and frequency modulation rate were similar to their own calls they would spend more time hiding after they fled.”
“Fairy wrens need to quickly flee if they think they’ve heard an alarm call. Once they are in the safety of cover they can decide if it was an alarm and if it’s safe to come out a bit earlier.”
“Interpreting these acoustic properties correctly is quite important. They don’t want to make a mistake – it’s always better to flee if they think it was an alarm. But if they flee when they hear any noise, they will never get anything else done.”
To test the idea further, Pamela generated synthetic calls based on the aerial alarm calls of the New Holland honeyeater, another bird in the neighbourhood whose alarm calls the fairy wrens respond to.
“The alarm call of a honeyeater is quite different to that of a fairy wren. It has no frequency modulation at all, is very short in duration and has a natural peak frequency of about 4 kHz, whereas the fairy wren alarm call sits at about 9 kHz.”
Pamela found that fairy wrens not only responded to synthetic calls when they had the peak frequency of honeyeater calls, but also responded when synthetic calls had the peak frequency of fairy wren calls. This is despite the structure of these synthetic calls being based on honeyeater alarm calls and completely different to that of a fairy wren alarm calls.
Fairy wrens in the Botanic Gardens interpret synthetic calls within a range of peak frequencies as signals of danger, and these peak frequencies are shared by alarm calls of other species they are familiar with. Responding to calls which have similar acoustic properties to their own alarm calls increases a fairy wren’s chances of picking up on danger when it is present.
“If a fairy-wren encountered a new species, and their calls had a similar peak frequency to their own alarm calls they would probably respond to them,” Pamela thinks.
“They don’t necessarily need to learn responses to the calls of other species as long as they have a similar peak frequency to their own alarm calls.”
So next time you hear the cacophony of bird calls, remember, you aren’t the only one keeping an ear out.
By Casey Hamilton