Birdsong helps scientists to count bird populations
By Pallab Ghosh
Science correspondent, BBC News
Ecologists have developed a way of monitoring the sizes of bird populations by recording their song.
Writing in the Journal of Applied Ecology, the team says the new technique gives a more accurate estimate of numbers.
Previously, researchers used nets to capture the birds, which can also be stressful for the animals.
The research team monitored the chirping of the ovenbird - a small warbler found in North America.
The researchers recorded the birdsong using four microphones. They combined the sound information and used a computational method to convert it into an estimate of the density of birds in the area.
According to Dr Deanna Dawson of the US Geological Survey (USGS) the ovenbird was the ideal species for testing out the new method.
"We chose [them] because they have a very distinctive call," she said.
"They're quite loud and most of the time they're on the ground or quite low in the forest canopy."
But the researchers say their technique could be used to study any songbird.
Grahame Madge from the Royal Society for the Protection of Birds (RSPB) welcomed the development.
"At this time more than any other time in history, it's vitally important that we have information on bird populations across the UK and the rest of the world," he said.
"About one in eight of the world's birds are facing extinction, so having the best available information about how they are doing year-by-year is vitally important to their conservation."
And, according to Mr Madge, the ear is often more useful for conservation research than the eye.
"Researchers in the UK are already using the songs of birds as a way of counting the population," he said.
"One example is the bittern. It lives in dense reed beds and it has an amazing booming call. Males apparently can be heard for up to 3km (1.8 miles) on still mornings.
"By getting a fix on where each of the males are calling from in these reed beds, we can begin to build up a map of where the birds are."
In the UK, scientists have a fairly accurate picture of trends in bird numbers - largely because of the efforts of thousands of highly skilled bird watchers - who report their sightings to the British Trust for Ornithology.
But, according to Dr Andy Musgrove, who works for the trust, the new approach could still be useful, particularly in areas that are difficult to access, such as reed beds.
Dr Musgrove's colleague, Dr David Nobel believes that the sweeter the bird song, the more useful the system might be.
"This would be useful for birds with complex songs, such as the nightingale," he said.
"It's easier to recognise individuals using this technique and you'd also be more certain, while you are using this technique, that you have a good proportion of the population singing."
And Dr Nobel believes that the system would be especially useful in countries where there is less experience in monitoring bird numbers.
"You have expedition type work in countries where we know very little about bird populations," he said.
"You can imagine that this might be something to add to the information that you are getting through other techniques - so there's lots of potential for this method."
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