Dawson DK, Efford MG 2009. Bird population density estimated from acoustic
Journal of Applied Ecology 46: 1201–1209.
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
there are few rigorous methods for estimating population density
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
methodology that addresses jointly the second and third parts of
the problem. We have extended
SECR to use uncalibrated information
fromacoustic signals on the distance to each source.
3. We applied this
extension of SECR to data from an acoustic survey of ovenbird Seiurus
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
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).
fitted model predicts sound attenuation of 0.11 dB m–1 (SE 0.01 dB m–1)
in excess of losses from
5. Synthesis and
applications. Our method for estimating animal population density from
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
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
recording interval, or that the per capita rate of vocalization is known. We
requirements can be met, with suitable field methods, for a
significant number of songbird species.