Acoustic Device Advancement Encourages a Wide Variety of Monitoring Improvements

By Shawn Carroll

If a bird chirps in a forest and no one is around to hear it, does it make a sound? WRA is about to find out! In late 2020, WRA avian biologists Shawn Carroll and Nick Wagner kicked off an exciting pilot project by successfully deploying five AudioMoth devices in coastal sage scrub habitats at the Soquel Canyon Mitigation Bank (Bank) in Chino Hills, California. The AudioMoth devices are considered a cost-conscious alternative to expensive acoustic monitoring devices typically used for biological monitoring, without sacrificing sensitivity or functionality. Funding for the project is being provided by the Bank Sponsor, Land Veritas and by WRA’s internal technical innovations grant program, which fosters research and development for a wide variety of new and improved products, service offerings, and technology for the benefit of our clients and project partners.

An AudioMoth device deployed at Soquel Canyon Mitigation Bank.

Designed by Open Acoustic Devices, AudioMoths are open-source, low-cost acoustic loggers capable of recording both audible and ultrasonic sounds made by anything from birds and bats to frogs, mice, and crickets. They’ve even been used to monitor for the sound of gunshots and chainsaws in areas where illegal poaching and logging is a concern. AudioMoths have some advantages over similar portable acoustic devices, namely their small size (the specially-designed case measures just under 3 x 2.75 x 1 inches), low power consumption, and low cost (similar commercial devices cost at least five times more). While there is some trade-off in sound quality when compared to more expensive acoustic devices, the low cost of AudioMoths enable researchers to deploy more units over larger areas and extended periods of time, presenting long-term monitoring opportunities that would have been cost-prohibitive in the past.

This sonogram of a Bewick’s wren song was recorded with an AudioMoth and analyzed in the Kaleidoscope Pro software.

The AudioMoth devices will allow WRA’s biologists to record bird songs for up to 360 hours at a time on one set of batteries – nearly two months running during the peak hours of the morning songbird chorus. Once that data is recorded and the memory cards collected, the recordings will be processed through specialized software (Kaleidoscope Pro by Wildlife Acoustics, Inc.). The software can automatically distinguish individual notes and phrases and classify similar calls together for manual review. Once the songs of species of interest are identified, that data is used to classify the remaining thousands of hours of calls and songs – greatly reducing the time needed for a biologist with good ears to analyze the recordings. While the main goal of the year-long project is to establish presence of the threatened California gnatcatcher (Polioptila californica), biologists hope to use the data to identify new species at the Bank and to study the seasonal variation in bird activity and species composition.

The California gnatcatcher, a threatened species under the Federal Endangered Species Act, is found in throughout coastal southern California and Baja California. Populations declined throughout the 20th century as urban sprawl reduced habitat available for the species. (Photo courtesy USGS/Public Domain)

WRA biologists are also planning to test the AudioMoth devices in the San Francisco Bay Area to listen for the kek calls of the endangered California Ridgway’s rail (Rallus obsoletus obsoletus) and the high-pitched vocalizations of the endangered salt marsh harvest mouse (Reithrodontomys raviventris).

If these initial studies are successful, the relatively low cost of AudioMoth devices compared to more expensive, commercial devices means that projects using acoustic recorders, such as this, are no longer cost prohibitive. WRA hopes to be able to implement long-term, remote monitoring on future projects in isolated or seldom-visited locations for species inventory, presence/absence surveys, and endangered species detection – and to see if birds really do sing when no humans are around to hear them.