Birds communicate by singing or calling, and biologists
have long counted these cues to get an index of bird abundance.
Photo: S Siva
Saravanan
http://beta.thehindu.com/news/article56270.ece
Ecologists have at last worked out a way of using
recordings of birdsong to accurately measure the size of bird populations.
Developed by a joint team from the US Geological Survey
and
Birds communicate by singing or calling, and biologists
have long counted these cues to get an index of bird abundance. But it is much
harder to work out the actual density of a bird population because existing
methods need observers to measure either the distance to each bird, or whether
they are within a set distance from the observer.
This is straightforward if birds are seen, but
difficult when birds are heard but not seen.
Team leader Dr Murray Efford said a way has been found
to estimate population density of birds or other animals that vocalise by
combining sound information from several microphones.
A sound spreading through a forest or other habitat
leaves a ‘footprint’. The size of the footprint depends on how quickly the
sound attenuates.
Mathematically, there is a unique combination of
population density and attenuation rate that best matches the number and ‘size’
of the recorded sounds. Computer methods were used to find the best match and
thereby estimate density.
The researchers developed the method by recording the
ovenbird—a warbler more often heard than seen—in deciduous forest at the
Patuxent Research Refuge near
“Sound intensity and other characteristics can be
measured from the spectrogram—the graph of the sounds—to improve density
estimates. Archiving the sounds also makes it possible to re-examine them, or
to extract additional information as analytical methods evolve,” he said.
The findings have been published in the Journal of Applied Ecology.