Ecologistas descobrem nova solução para o monitoramento de espécies crípticas

Ecologists Sound out New Solution for Monitoring Cryptic Species

ScienceDaily (Nov. 28, 2009) — Ecologists have at last worked out a way of using recordings of birdsong to accurately measure the size of bird populations. This is the first time sound recordings from a microphone array have been translated into accurate estimates of bird species' populations. Because the new technique, reported in the British Ecological Society's Journal of Applied Ecology, will also work with whale song, it could lead to a major advance in our ability to monitor whale and dolphin numbers.

Developed by Deanna Dawson of the US Geological Survey and Murray Efford of the University of Otago, New Zealand, the technique is an innovative combination of sound recording with spatially explicit capture-recapture (SECR), 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. This is straightforward if birds are seen, but difficult when birds are heard but not seen.

According to Dawson: "We devised a way 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. We used computer methods to find the best match, and thereby estimate density."
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Journal Reference:
Deanna K. Dawson and Murray G. Efford. Bird population density estimated from acoustic signals. Journal of Applied Ecology, 2009; DOI: 10.1111/j.1365-2664.2009.01731.x

Bird population density estimated from acoustic signals

Deanna K. Dawson*1 and Murray G. Efford 2

1 USGS Patuxent Wildlife Research Center, 12100 Beech Forest Road, Laurel, MD 20708, USA and 2 Zoology Department, University of Otago, PO Box 56, Dunedin 9010, New Zealand

*Correspondence author. E-mail: ddawson@usgs.gov

KEYWORDS

acoustic localization • bird counting • density estimation • microphone array • passive acoustic methods • sound attenuation • spatially explicit capture–recapture

ABSTRACT

1. Many animal species are detected primarily by sound. Although songs, calls and other sounds are often used for population assessment, as in bird point counts and hydrophone surveys of cetaceans, there are few rigorous methods for estimating population density from acoustic data.

2. The problem has several parts – distinguishing individuals, adjusting for individuals that are missed, and adjusting for the area sampled. Spatially explicit capture–recapture (SECR) is a statistical methodology that addresses jointly the second and third parts of the problem. We have extended SECR to use uncalibrated information from acoustic signals on the distance to each source.

3. We applied this extension of SECR to data from an acoustic survey of ovenbird Seiurus aurocapilla density in an eastern US deciduous forest with multiple four-microphone arrays. We modelled average power from spectrograms of ovenbird songs measured within a window of 0·7 s duration and frequencies between 4200 and 5200 Hz.

4. The resulting estimates of the density of singing males (0·19 ha−1 SE 0·03 ha−1) were consistent with estimates of the adult male population density from mist-netting (0·36 ha−1 SE 0·12 ha−1). The fitted model predicts sound attenuation of 0·11 dB m−1 (SE 0·01 dB m−1) in excess of losses from spherical spreading.

5. Synthesis and applications. Our method for estimating animal population density from acoustic signals fills a gap in the census methods available for visually cryptic but vocal taxa, including many species of bird and cetacean. The necessary equipment is simple and readily available; as few as two microphones may provide adequate estimates, given spatial replication. The method requires that individuals detected at the same place are acoustically distinguishable and all individuals vocalize during the recording interval, or that the per capita rate of vocalization is known. We believe these requirements can be met, with suitable field methods, for a significant number of songbird species.

Received 19 December 2008; accepted 9 October 2009 Handling Editor: Esteban Fernandez-Juricic

DIGITAL OBJECT IDENTIFIER (DOI)
10.1111/j.1365-2664.2009.01731.x About DOI

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