On September 1, 2018 the Nipissing Naturalists Club hosted an evening bat walk in Laurier Woods led by FRi’s Species at Risk Biologist, Rebecca Geauvreau. Participants braved the impending rain and high humidity for the opportunity to see and ‘hear’ Ontario’s only flying mammal. We used hand-held bat detectors (Wildlife Acoustics Echo Meter Touch) which translate the ultrasonic bat calls into lower frequency sounds audible to human ears.
There are eight (8) bat species in Ontario and all of them eat insects exclusively. They are often spotted foraging over wetlands and shallow waterbodies, along forest edges and trails, and near homes and cottages. Bats can be found in almost any habitat; on this night we recorded big brown bats, silver-haired bats and hoary bats foraging over the wetlands.
Bats use echolocation to navigate, find food and access drinking water. Echolocation is a pattern of calling and listening in the ultrasonic frequency range. Humans talk and listen in the 200 hertz (Hz) to 5 kilohertz (kHz) range, bats call in the range of 14kHz to 90kHz or higher!
Close-up of digital output from Echo Meter Touch
Bats call and then listen for the echo bouncing off nearby objects. They use the echoes to determine an item’s size, shape, distance and texture. This calling and listening pattern is useful up to a few metres in front of the bat. This explains their apparently erratic flight when foraging.
High frequency calls tell the bat “what’s immediately in front of my nose” while the lower frequency calls answer the question of “what’s out in front of me”. Bats emit a short pulse of sound, quickly running through many frequencies. As they get closer to a prey item, bats issue a series of quick pulses known as a ‘feeding buzz’.
Bat Pulses, Passes and Calls - Spectrograms
A spectrogram is a picture of sound through time. The following bat spectrogram shows a series of big brown bat pulses as it ‘sees’ and pursues an insect, ending in the fine-tuning buzz that happens right before it catches and eats it’s prey.
Feeding buzz registered at the right side of the recording above.
Slower-flying bats tend to forage in high clutter environments and typically emit higher frequencies, while faster flying bats tend to forage in the open and emit lower frequencies – because they need to detect objects in the distance.
Echolocating takes energy – frankly it’s exhausting! It’s tied to respiration & wing beat. Think about standing with your arms at side and letting out a scream. Now think about the difference (less effort) required to scream when you ‘flap’ your arms. Bats synchronize wing beats with echolocations to maximize the strength of their call and minimize their effort.
Emitting ultrasonic frequencies is only half of echolocation – the other part is listening. As noted, echolocations are like screaming; they very loud. Bats can temporarily disconnect their ear bones during the pulse so that their hearing is not damaged.
Bats in Your Backyard
Bats are still (thankfully) generally widespread on the landscape. Ontario’s bats will be foraging on most fair-weather nights between April and October. Choose a comfortable spot in an open area and look to the sky around dusk. Whether you live in an urban area with sidewalks and streetlights or in a rural landscape with fields and forest, there are bats near you.
Photo Credits: Kaye Edmonds