The great gray owl can find and catch mice hiding under up to 50 cm of snow. Thanks to the work of a team of researchers, we finally know how these birds of prey can accomplish such a feat.
According to the new studyposted on November 22 in Proceedings of the Royal Society bWhen hovering over snow, owls use their broad faces to locate muffled sounds emitted by their prey.
“Snow is known to absorb sound,” says the study leader. Christopher Clarkan ornithologist at the University of California, Riverside, who this year conducted a series of experiments aimed at measuring sounds in the province of Manitoba, Canada.
Prior to this study, researchers believed that birds of prey focused on ultrasound emitted by the rodents’ vibrations. But owls also seem to be able to pick up on lower-pitched sounds, such as those made by mice when tunneling in the snow.
Although owls’ ears are often thought of as being on the top of their heads, they are actually closer to the center of their faces. It is bordered by a ring of feathers that reflect sounds and direct them towards the ears of the animal.
The larger the owl’s facial disc, the greater its ability to hear lower frequencies. According to Clark, the great gray owl, which is found throughout the northern hemisphere, has the largest facial disc of any owl species.
We think the reason their facial discs are so large is to make them more sensitive to low-frequency sounds. »
In February 2022, Clark and his colleagues traveled to the Manitoba forests and discovered seven recent craters; These holes are excavated by owls when they dive into the snow to catch their prey.
The team drilled an extra hole next to each existing hole and placed the speakers there. With freezing temperatures reaching -30 degrees Celsius, the researchers had to deal with technical issues. “It was an exhilarating job,” Clark says, “the job kept going badly because of the weather.”
The team then used an acoustic camera, equipped with an array of microphones, to record various noises in the environment. She then played white noise (high-frequency sound), and Foley (low-frequency sound) recordings through loudspeakers.
By manipulating the layers of ice on the speakers, the team was able to assess the effect of snow depth on sound frequencies. The data revealed, for example, that while much white noise could pass through 20 cm layers of ice, only low-frequency sounds managed to pass through 50 cm thick layers; It is precisely these sounds that owls can detect.
Clark and his colleagues then studied vocal mirages, an acoustic effect owls experience when hunting.
When sound waves coming from underground hit the surface of the snow, their path bends. Because of this phenomenon of sound distortion, called refraction, the source of the sound seems to come from a different place than where it is actually emitted: therefore, unless the owl is directly above its prey, it fails to locate it in the middle. from the snow patch.
“It’s the same problem we have when trying to pick things up underwater,” he says. Megan Galla sensory ecologist at Vassar College in Poughkeepsie, New York, who was not involved in the study.
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