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13 December 2007

Call of the Wild 1


DR KIRSTEN PARRIS: We're researching the effects of traffic noise on the calling behaviour of birds. Normally, if there was no background noise from roads or other human-generated noise, they'd be able to hear each other at least 100 metres apart. But where we have loud traffic, it's likely they can only hear each other 20 or 30 metres apart. Traffic can be very noisy, depending on how many cars there are on the road and how fast they're travelling and how many big trucks there are. So at the largest roads that we've included in our study, there are about 150,000 cars coming past a day and that makes a noise, if you're right next to it, of up to 95 decibels, which is loud!

ANGELA SCHNEIDER: We're looking in a lot of different ranges of sites, so sometimes we're recording next to freeways, which can be really busy and it makes it really difficult to hear the birds. And other sites are very quiet, where there might be, you know, one car every hour. The best time to record is at dawn but you can also get some recordings at dusk, and that's the most active time for birds.

DR KIRSTEN PARRIS: We can analyse the frequency of the different notes in the calls. So for example, this is the grey fantail - how it would sound at a quiet site. And if I focus in on this particular note... this is the lowest tone - it's about 4,500 hertz. So if we now go to a recording of the same bird at a noisy site, it sounds like this. And if we zoom in on our lowest note and analyse that, it's about 5,100 hertz, so that's a change of 600 hertz. That means that the birds are calling at a higher...a higher pitch, they're changing their tune where it's noisy, and that's the interesting finding of this research.
This is a very noisy site close to a busy road. It's pretty loud, and it's difficult for you to hear me speaking over this noise, so you can understand that the birds have trouble hearing each other over this kind of noise too.
The birds aren't changing their tune very much, so on average, they're only going up one note on the musical scale. For example, with the grey shrike-thrush, it goes from here to here. So changing their tune from there to there increases the distance they can be heard by about half a metre. So they'd really need to be up here or in fact right off the end of the piano to be able to be heard properly over the traffic noise.
We here have so far only studied two species of birds, the grey shrike-thrush and the grey fantail. But there have been a number of studies done elsewhere in the world in Europe and in North America and they've found very similar results. So across five or six species of birds now, this is starting to become an established pattern. It's a problem because communicating acoustically with sound is really a very important thing for birds. It's a matter of life and death. If they can't hear, in the first place they may have trouble attracting mates and breeding and keeping the population going. And also, if they can't hear each other's warning calls, they may be more likely to be taken by predators. Wherever you are, wherever there are roads and wherever there are birds, there's likely to be this problem. The birds will have difficulty hearing each other and they may be responding in the only way that they can.