Contributions of environmental conditions and sound characteristics to differences in perceptibility: Recommendations for passive acoustic monitoring

Passive acoustic monitoring is increasingly used in ecological research to
study a wide variety of taxa; however, properly accounting for
perceptibility, a component of detection probability, has been challenging
because sound attenuation is influenced by numerous variables in the
surrounding environment, and the rate of sound attenuation is not
typically quantified. We used sound playback experiments to investigate
the effect of several environmental variables, including vegetation type
and weather, on the perceptibility of a variety of sounds and species. We
also investigated species-specific variables such as sound frequency,
syllable rate, and bandwidth. We quantified the amount of bias in
perceptibility resulting from species sound characteristics and vegetation
type, and estimated perceptibility and the area surveyed for different
combinations of species and habitat type. We found that distance was the
strongest predictor of perceptibility, but that vegetation type
significantly influenced perceptibility, particularly when comparing open
versus closed vegetation classes. Sound frequency and bandwidth of species
were also important predictors of perceptibility. We found that ignoring
variation in perceptibility due to excess sound attenuation from
vegetation could bias area surveyed during acoustic surveys by up to 400%
(mean = 167.4%). We conclude that other than distance, perceptibility is
influenced most by species-specific vocalization traits and by the habitat
these vocalizations transmit through. We provide recommendations for
incorporating these findings into future research to account for varying
perceptibility and improve the accuracy of bird monitoring and research.