Scientist have developed a way of estimating bird populations without netting them - through data from a microphone array.
The technique, which can be extended to whales and dolphins, is a sonic version of 'spatially explicit capture-recapture' (SECR), which relies on statistical analysis of data from animals that are capture, marked, released, and recaptured along with other unmarked animals.
Sound-based SECR has been developed by Deanna Dawson of the University of Otago.
"A sound spreading through a forest or other habitat leaves a footprint. The size of the footprint depends on how quickly the sound attenuates," said Dawson. "Mathematically, there is a unique combination of population density and attenuation rate that best matches the number and size of the recorded sounds. We used computer methods to find the best match, and thereby estimate density."
Dawson and Efford developed the method by recording the ovenbird (Seiurus aurocapilla) - an illusive creature of the woods of Maryland with a distinctive song, and whose males sing from lower forest layers. Its nest look like a 'Dutch oven' cooking pot.
They used four microphones close to the ground in a square with 21m sides. Over five days, they moved the microphones to 75 different points across their study area and recorded singing.
Further analysis of recordings can improve accuracy. "Sound intensity and other characteristics can be measured from the spectrogram to improve density estimates. Archiving the sounds also makes it possible to re-examine them, or to extract additional information as analytical methods evolve," said Dawson.
A paper 'Bird population density estimated from acoustic signals' has been published in the Journal of Applied Ecology.