Now, with some lateral thinking and collaboration with scientists in the United States and Scotland, he has been able to adapt the system to count birds and says it could be used to tally any land and sea creatures, including endangered species.
The scientists' work, reported on Friday in the British Ecological Society's Journal of Applied Ecology, is being regarded as a major advance because it enables scientists to monitor bird populations using an acoustic technique without having to physically catch and release specimens.
Dr Efford, an adjunct senior lecturer in the University of Otago's zoology department, said yesterday it had been fun and satisfying working on the project.
"You beaver away in a corner, and if you can find something which is generally useful it is an unexpected bonus."
The technique developed by Dr Efford and Deanna Dawson, of the US Geological Survey, is a combination of recording bird songs from multiple locations and applying a mathematical formula to the information to calculate the number of birds in a given area.
A sound spreading through a forest or other habitat left a "footprint", Dr Efford said.
The size of the footprint depended on how quickly the sound attenuated (weakened).
Mathematically, there was a unique combination of population density and sound attenuation rate which best matched the recorded sounds.
Computer methods were used to find the best match, and thereby estimate population density, he said.
Statistician David Borchers, from St Andrews University in Scotland, helped refine the computer modelling.
Dr Efford and Ms Dawson have been trialling the system for about five years with the ovenbird, a warbler more often heard than seen in the forests of Maryland in the United States.
When compared with the standard method of netting birds to count them, the new acoustic technique was found to give a more accurate estimate of bird numbers, he said.
Potentially, the acoustic technique could be adapted for hydrophones, underwater microphones used to measure hard-to-reach populations of marine mammals such as whales and dolphins, he said.
"Developing ways of estimating whale and dolphin numbers acoustically is seen as critical for understanding these species' populations."
Dr Efford said he hoped the technique could also be used to gain a better understanding of New Zealand endangered bird species, such as the mohua.
