Field tests of small autonomous recording units: an evaluation of in-person versus automated point counts and a comparison of recording quality

ABSTRACT

The proliferation of small autonomous recorders makes it easier than ever to sample terrestrial acoustic animals and soundscapes. I conducted a comparison of four small recorders to evaluate their performance in a field setting: Wildlife Acoustics Song Meter Mini; Wildlife Acoustics Song Meter Micro; Open Acoustics Audiomoth; and Cornell SwiftOne. I address two questions: (1) How do in-person point counts compare to recorder-based point counts using these small autonomous recorders? (2) How does the quality of the recordings compare across these small autonomous recorders? To evaluate the performance of the recorders in point counts, I conducted in-person and recording-based point counts at ten locations. Each of the recorders performed similarly well at point counts, producing comparable estimates of species richness, although all of the autonomous recorders under-estimated species richness. To evaluate recording quality, I conducted a sound transmission test, broadcasting and re-recording sounds. Recorders varied in their frequency response above 12 kHz, but showed only subtle differences in the frequency response at frequencies below 12 kHz. I conclude that each of these types of small recorders provide bioacousticians with useful tools for conducting point counts, and for passive monitoring of animal sounds, with only subtle differences across the investigated models.

KEYWORDS:

• ARUs
• autonomous recorders
• biodiversity assessment
• digital recorders
• PAM
• passive acoustic monitoring
• point counts
• recording quality
• sound transmission

Acknowledgements

I thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding my research programme through Discovery Grants and Research Tools and Instrumentation Grants. I thank the Bowdoin Scientific Station for logistical support during the field component of this research; this is publication 291 from the Bowdoin Scientific Station. I thank S. Doucet for randomising and renaming the sound files so that I could conduct the analyses anonymously to recorder type. I thank N. Shangi who compared the setup time for the devices from the perspective of a naïve user. I thank C. Cross for providing the loudspeaker for playback in the transmission test. I thank past and present members of the Mennill Sound Analysis Lab for collecting recordings that were used in the sound transmission tests. I thank present members of the Mennill Sound Analysis Lab and two anonymous reviewers for comments that improved the manuscript.

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/09524622.2024.2315054

Disclosure statement

I purchased all of the recorders at full price using my research grant. None of the manufacturers of the recorders were aware that I was purchasing the recorders to be part of this comparison. As an active researcher in the field of bioacoustics, I have relationships with two of the manufacturers. In 2004-2005, I conducted postdoctoral research at the Cornell Lab of Ornithology, which produces the SwiftOne recorder. Since 2022, Wildlife Acoustics has collaborated with my lab on a Mitacs Accelerate project studying nocturnal flight calls of migratory birds, led by a postdoctoral researcher in my lab. In both cases, I feel that my affiliation with the Cornell Lab of Ornithology and Wildlife Acoustics does not compromise my ability to objectively evaluate the recordings. In particular, my analyses were conducted with the type of recorder anonymized.

Additional information

Funding

This work was supported by the Natural Sciences and Engineering Research Council of Canada.