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 University of Otago, the technique is an innovative combination of sound recording with spatially explicit capture-recapture, a new version of one of ecologists’ oldest tools for monitoring animal populations. 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.
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