Music to our ears…if only we could hear it! Recording bat calls to mitigate impacts to bat species in El Centro Field Office
By Ismael Ramirez, El Centro Field Office Natural Resource Specialist and Peter DeJongh, El Centro Field Office Wildlife Biologist
To the human ear, bats make a pinging or clicking noise. However, bats communicate with one another on a more detailed level. They make high-pitched sounds at frequencies up to seven times higher than humans can hear and use echolocation, bouncing sounds off distant objects, to “see” and socially interact.
Every winter, staff from the BLM’s El Centro Field Office in southern California conduct bat outflight surveys in the Cargo Muchacho Mountains in Imperial County, a center of historic mining activity. But this year, the BLM added ultrasonic acoustic detectors to the openings of abandoned mine shafts to monitor bats by recording their unique calls.
Old mining shafts and openings in the mountains provide great roosting habitat for multiple bat species. Several Bureau Sensitive Species, ones that have been designated by the BLM State Director as at-risk, live in these mines, including California leaf-nosed bats (the most common), pallid bats, and Yuma myotis.
To conduct the outflight surveys, surveyors use infrared lights and night vision goggles to set up in front of horizontal mine entrances called “adits,” or on a “cupola,” which is a gate covering a vertical mine shaft. For more than an hour, surveyors will count the number of bats flying in and out of the mine shaft openings. The outflight surveys gather information on the size and activity of bat populations in each mine feature.
Ultrasonic acoustic detectors, which consist of a 12-foot pole secured by three strings with a microphone at the top, are set up next to the openings of the old mine shafts and left overnight to record the bats’ ultrasonic calls. Bat calls are unique and help the BLM identify which species are present in the area during a specific time of year and which species of bats utilize the abandoned mines.
Bat outflight surveys are only conducted in the winter and summer months to assess how population and activity change between seasons; however, acoustic monitoring can take place year-round. The BLM uses this data to better understand bat movements and behaviors, and to better inform the management of bat species and their habitat. Gold mining continues in the Cargo Muchacho Mountains and the data will help evaluate the impacts of mining activities and develop methods to mitigate or eliminate impacts to these important animals.