Acoustic and Ultrasonic Monitors
Listening for Birds and Bats Calling in the Night
The U.S. Fish and Wildlife Service has operated over 30 acoustic and ultrasonic (SM2Bat+, Wildlife Acoustics) monitors at almost 100 locations around the Great Lakes. These monitors listen for the audible chirps (acoustic) that birds make as they migrate and the echolocation calls (ultrasonic) that bats make as they feed and navigate.
Acoustic/Ultrasonic Monitor - Photo by Becky Horton/USFWS
Acoustic Monitor Setup- Photo by Becky Horton/USFWS
These acoustic monitors have a range of up to 100 m and operate from approximately dusk to dawn, recording all the calls that they hear. The white cup around the ultrasonic microphone and the plexiglass plate underneath the acoustic microphone help to filter out noise from the wind, insects, and other animals on the ground. Once the data is collected, the recordings are scanned by software that filters out the calls and helps identify them by their specific shape in a spectrogram. Working with Kevin Heist at the University of Minnesota and Doug Johnson with the USGS, we can compare our data with studies at other locations around the United States. Kevin filters out calls from both our data and his data and when examined, many sites around the Great Lakes have magnitudes higher bat passes per night than elsewhere.
Some of these acoustic monitors are paired with the avian radar units and the weather stations that accompany them. By grouping these two different methods, we can understand a bigger picture of what is happening with migration. Some examples of this are determining what environmental conditions affect bat activity, such as wind speed and temperature, and determining times when only birds or bats are active and comparing that with the radar data to try and determine their unique radar signatures.
Bird calls (below left) are used to keep a flock together during migration at night. Bat echolocation calls (below right) are used for navigating the environment and locating food. Many bat calls have a specific shape that can be identified to the species that created it. Bird calls are harder to identify, but researchers are working on identifying these to species or guild.
Acoustic monitors set out by this project around the Great Lakes have recorded magnitudes more bat passes than inland locations such as southern Minnesota, Wisconsin, Indiana, Pennsylvania, and Texas that used the same equipment and were also analyzed by Kevin Heist at the University of Minnesota. This may indicate greater bat activity along the shorelines of the Great Lakes than many other places around the country.
Below is a movie showing the numbers of bat calls at the monitors located along southern Lake Erie for a short period of the spring season of 2012. This video is a subset of the data and was shortened to provide easier viewing. Larger circles indicate higher amounts of bat passes. Some nights show a pattern of activity across all of the monitors, likely indicating good weather for bats to move and feed. Other sequences of nights show only activity in one area, sometimes moving from one monitor to another in sequence. Movement like this may indicate bats migrating from one area to another on their way to their summer habitat.
We would like to thank all of the volunteers and organizations that have helped maintain the acoustic monitors throughout our project. Without them we could not have collected data from such a wide area.
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